As one of the few traditional housewives left in her Arlington County neighborhood during the day, Mary Hynes finds she is much in demand.

Besides caring for her own four children, she gets called on to bake cookies for the local elementary school's special functions, make the flyers sent home by the PTA and drive other children to school hearing tests. Invariably she's the one her friends turn to to play back-up parent for their kids.

"I'm the person people write down: 'If an emergency, call Mary Hynes,'" she says. "There are some days when I'm the only one home, and they all call me. I've had up to 10 children here."

The reason Hynes's neighborhood seems emptier and her responsibilities greater is simple: Most of the women who live nearby have jobs and are gone all day.

In the past two decades millions of women seeking self-fulfillment or out of economic necessity have entered the work force, a development that has had a profound impact on everything from child rearing to fashion. But perhaps nowhere can the far-reaching effect of working women be seen better than in communities like Arlington, where the absence of the traditional homemaker has shattered the stereotype of the suburban housewife and altered some fundamental aspects of suburban life.

The suburbs were designed around the concept of the husband as breadwinner and the wife as homemaker. Suburban planners, almost always men, paid scant attention to public transportation, day care and other support services for working mothers. They counted on the "moms" to be there, with the family station wagon, chauffeuring the kids to school and playground activities while juggling a dizzying agenda of household chores.

But the days "when Ethel was always home" are over, says a Census Bureau official, referring to the "I Love Lucy" television show of the 1950s. "Yesterday's next door neighbor is probably out working herself."

Today in the Washington suburbs, according to Census figures released this week, there are now more women--63 percent--going to work than staying home, an increase of 15 percent over 10 years ago. During the same period the number of working women in the District of Columbia increased by a much smaller proportion--from 56 to 59 percent.

In the Washington area as a whole, 63 percent of all women--767,901 out of 1,237,881--now work, the highest percentage of any metropolitan area in the country.

"It's a national suburban phenomenon," says Hugh Wilson, director of the Institute for Suburban Studies at Adelphi University in New York. He says suburbs throughout the country are now dominated by two-income families, a sociological development that has had significant implications for parents, children and the communities themselves.

With many "moms" no longer at home, working parents are reorganizing their time and family responsibilities, joining with other parents in sometimes elaborate babysitting and carpooling arrangements and redefining child care and household roles. Their children are growing up in day-care centers or else spending so much time alone that some jurisdictions now offer special "latch key" programs that teach unsupervised children first aid and simple cooking.

Suburban communities face other consequences as well. Empty houses and unsupervised teen-agers have contributed to the rise in burglaries and crime. Demand has increased for costly day-care facilities. And school, church and charity organizations have found it increasingly difficult to recruit volunteers because the pool of housewives they have always relied on is getting smaller and smaller.

Through it all, the pressure on women who are balancing a home and work life is enormous--and not at all what their mothers probably experienced when they gave their lives over to their children and chose the isolated but open-spaced suburbs as the ideal place to rear them.

"I have this feeling that I have to be constantly organized and efficient with my time at work and on weekends just to fit it all in," says Marty Treadwell, of Vienna, who has returned to work after 12 years at home with her three children.

Now, in addition to a full-time job as meetings and convention planner for a trade association, Treadwell, 39, still shops, supervises the household chores and participates in three different carpools. When it's her turn, she does without lunch so she can get off work early to pick up her kids and others from soccer, swimming and music practice. The former president of the Fairfax County League of Women Voters also tries to keep up on some of the volunteer activities that used to occupy so much of her time when she was at home.

"There's never a point in time where I just sit down, put my feet up and stare at television, but then there wasn't much of that when I was at home, either," Treadwell says.

Working women in the District, of course, share many of the same problems as women in the Virginia and Maryland suburbs. But there are some differences.

For one thing the longer distances in the suburbs make a car a necessity, and spending time getting to work or taxiing children is unavoidable. For another, families often move to the suburbs because they want their children to have better schools, more recreational facilities and open space and so feel particularly pressured to help their sons and daughters take advantage of those opportunities.

Despite hectic schedules--and the slowness of communities and employers to offer adequate support services--most working women, including divorced and single mothers, manage to cope with these challenges and new demands on their time. They also benefit economically. The high proportion of two-income families is one reason why the Washington metropolitan area has the highest median family income--$27,515--of any large metropolitan area in the county. Still, the decision to work has its negative effects.

The shortage of convenient, affordable and quality day care is a constant concern for working parents, so difficult, in fact, that many reluctantly end up leaving their kids to fend for themselves. School principals have reported cases where children, some as young as seven, six and even five, are left alone after school until their parents get home. Known as "latch key children," the youngsters are either ordered to stay in the house or else are out, roaming the streets, with nothing to do.

For children of low-income families, the problems are even more acute. Many don't have the outlet of Scouts or soccer because their mothers and fathers aren't able to be leaders or coaches. Jessica Reid, director of the after school Cora Kelly Recreation Center in Alexandria, says poorer parents sometimes need to work two jobs each just to make ends meet, "which leaves some of these kids on their own until 9 or 10 p.m. or even midnight."

Although the level of maturity varies with each child, child-care authorities generally set 10 years as the age under which children should not be left alone for extended periods of time.

Yet area jurisdictions report their day-care programs, libraries and recreation centers are jammed after school with youths who have no place else to go or are waiting for their parents to pick them up. Often, children are still there when the facilities close. And, for every child in the library doing homework, there are five others hanging out in less wholesome surroundings, according to police, who say juvenile crime is on the rise.

"What's happening to the kids is a very bad trend," argues Becky Allen, president of Montgomery County's Community Coordinated Child Care. She worries that unsupervised children are experimenting with alcohol, drugs and sex or becoming "t.v. boobs" because they don't know how to use their free time. But she also notes that even children in day-care facilities are "beginning to feel like prisoners" because they spend so much time in such facilities.

"Families are under a lot of stress just making the arrangements for day care , and there's an alarming number of families where no arrangements are being made at all," says Abby Sternberg, a social worker at the Mount Vernon Center for Community Mental Health in Fairfax County.

This situation, says Sternberg, can lead to feelings of abandonment. "If the child is left to make his own dinner and fix his cuts and bruises, he can get very angry and depressed."

The most serious consequence of this abandonment, according to area police, is juvenile crime. Youngsters left alone during the day know who's at home and who isn't, and some take advantage of this. In Prince George's County, for instance, police report that about 70 percent of the breaking and entering offenses there involve juveniles in their own neighborhoods.

Criminals, in general, have been paying more visits to the suburbs, according to police. Communities have countered by setting up their own 24-hour "Neighborhood Watch" programs, which police say have curbed some of the crime statistics. Yet here, too, according to a Montgomery County police official, it's getting harder to run the programs during the day because "there's not as many people home as we would like."

In Arlington County, retired Navy Department employee Irma McGraw, 67, is home during the day, and her presence is a comfort to her neighbors. She watches out for strangers and checks the block "if I hear any unusual noises."

Judith Rosen, director of Fairfax County's Office for Children, says a 1979 survey there showed 50 percent of the children under two years, 46 percent of the children aged 2 to 5 and 57 percent of the children aged 5 to just under 12 lived in households where all the adults worked.

"I would suspect it's higher now," said Rosen.

Gay Schneider, who runs the Westmoreland Children's Center in Bethesda, says her day-care program for younger children is pretty full. But her program for fourth, fifth and sixth graders is not, largely because many children this age look upon supervision as "kind of babyish" and pressure parents to "take the risk" of trusting them on their own.

But Schneider reserves her biggest concern for "the 10-year-old who is supposed to be taking care of his 6-year-old brother or sister--we see that a lot."

The "nobody's home" in the suburbs problem is also having its impact on community and charity groups, who are starting to lose their chief source of volunteers--the mothers.

"We traditionally depended on housewives, but most are out working and just don't have the time to volunteer," says Susan Share, director of the Northern Virginia Heart Association.

Reagan budget cuts have forced county governments to "beat the drums" for volunteers, according to one official. Yet there is seldom a pool of idle residents to call on.

"There's sort of this Montgomery County mystique of affluent people with time on their hands," says Charles Maier, director of county information and volunteer services. "People don't realize that much of that affluence is the result of two people working."

In Arlington County, Virginia DeSimone, president of the League of Women Voters, says her group and other League chapters are victims of their own success. Women who once honed their research and organizational skills there are now busy in the paid work force, so that "fewer of women's valuable hours are available to us."

Groups who rely on volunteers say they've started to tap the services of retirees or college graduates and women re-entering the work force who are looking for experience to put on their job resumes. More recently, groups in need have turned to working women and men, who can't give their time for long stretches, like many housewives, but can and do take on specific projects.

Still, women at home say they get called on most--because they are there. "I was called a lot more simply because I was at home all day," recalled an Arlington mother who now works part time. "Sometimes I wonder if it drives people to work."

But whether women work or stay home, they have each had to make their own individual adjustments to sustain that lifestyle.

Marilyn Seitz of Annandale went back to work after 10 years so that her family could afford a second car. It took her weeks to find a 9 a.m.-2 p.m. job that allows her to work without having to hire a babysitter for her children, age 9 and 7. Her kids now get themselves ready for school, get their own breakfast and put their own dishes in the dishwasher. Once a week, there's "mass sandwich making" for the freezer, so they can make their own lunches.

It's "a big change" for them, according to Seitz, 35. "I should have done it years ago."

Tricia O'Connor lives in the same Arlington neighborhood as Mary Hynes and says her community has become tighter because working and nonworking women there "depend on each other." Before her daughter, now 7, was in school, she relied on day care. Now she uses her in-laws to look after her son, 3, while she's away at work in the morning.

"Everything works fine as long as everyone's well," says O'Connor, who rates transportation as the most worrisome emergency facing working parents. When her kids need to be picked up or taken somewhere in an emergency, she's apt to turn to Hynes or others.

"Day-care centers don't provide transportation--neighbors do," says O'Connor, who likens the situation to "a bartering system--we do favors for each other."

Barbara Joseph, of Alexandria, found herself setting up two sets of rules for her children when she went back to work. Her son became "a latch key kid" when he was 10, left alone with the family dog from 2:30 to 5 p.m. "He had to call me when he got home, and he had a list of kids he could have in and a list of kids he couldn't."

But her teen-age daughter "was not allowed to have anyone in, period."

Carolyn Vale of Arlington works half days while her children, 7 and 9, are in school. "I work twice as hard because I still do all the car pools and piano hours and plays and soccer practices. Work is almost a relaxation period in a way because I can sit down for four hours."

Maritza Ergueta, an Arlington mother of a 5- and 8-year-old, says she couldn't have returned to work without her husband's aid around the house.

"We both help each other," says Ergueta, 30. "We had to really get well organized. Fridays are for cleaning, Saturday is for shopping and Sunday is for church and family activities."

Suburban studies expert Hugh Wilson warns that the pressures on working women will increase unless suburban communities accept the two-income family as a fact of life. He says local governments have been slow to meet the needs of working women because so many suburban leaders are men.

"Men are still the decision-makers, and most seem to come from the traditional families where the role of the woman is to stay home and take care of the kids," Wilson says. "They still have the idea that day care leads to marital break-up."

But Marty Treadwell, who went back to work to help pay the orthodonist bills for three daughters and to give them a role model, feels working has actually made her more accessible to her children than when she was a busy homemaker and volunteer.

"Now, I have one place and one phone number. They know where I am," Treadwell says. "Before, being home was better than a full-time job for less than no time pay." She prefers the paid work force.

"I'm not engaging in child neglect, I'm encouraging child independence," says Treadwell. Right now she feels her kids are becoming confident and capable because she has left the home. "But if I see any indications they are headed for trouble, I'll be back.