In a Northeast Washington classroom last week, 29 second- and third-graders, most of them 7 years old, were asked how many of them are sometimes left alone at home: All but two hands shot up.

Sitting at pint-sized chairs in a Bunker Hill Elementary School classroom, 10 in the group said they daily let themselves into an empty house with a key after they leave school at 3 p.m.

One second-grader explained why losing his key was not a problem -- he is small enough to go in through the doggie door. Once in, he plays Nintendo or visits with a neighborhood friend until his mother arrives about 7 or 8 p.m.

Increasingly, schools and social service agencies throughout the region have become aware of more -- and younger -- children left to supervise themselves, usually because their parents are working. Although they warn that these situations can be dangerous, child advocates say parents are caught between the need to work, the expense of child care and the fact that few have relatives or friends who can supervise children.

Last week, a 4-year-old girl was left alone for at least two hours in a Silver Spring apartment when she was ill. She was found, almost miraculously, after punching numbers randomly on her telephone and reaching a rural health department in Wisconsin, which kept the girl on the line for two hours while her location was traced. The girl said her mother had to go to work to get money, but the mother said she had taken the day off and had been running errands.

"We get calls from people who simply feel they have no choices," said Margaret C. Plantz, director of Project Home Safe, a national group that provides parents with information on preparing children to be home alone.

The problem of unattended children has prompted varying responses from government and nonprofit agencies.

The scene at Bunker Hill Elementary School was part of a session on safety tips for children left home alone, developed by the D.C. Hotline's PhoneFriend program and conducted in District schools for children in second through fourth grades.

"We are not endorsing the idea that children {this age} are home alone by themselves," said Channing Wickham, executive director of the D.C. Hotline. "We are responding to the fact that children are home alone . . . . We're trying to make the best of a situation that is no one's ideal."

Wickham said the PhoneFriend service was started about five years ago because the regular hot line was getting so many calls from latchkey children.

"Our volunteers were scared for the children," he said. The PhoneFriend line now gets about 40 calls a day, with the average age of the callers about 8. The group started its safety-skills program four years ago, with a goal of going into all D.C. elementary schools at least once a year.

D.C. Hotline worker Lauren Simpson says that typically half of the children in each session say they let themselves into their homes after school.

Child-safety advocates say it still is not common to find a 4-year-old by herself, as happened in the Silver Spring incident, but they said they are seeing more cases of children who are simply too young to be caring for themselves.

"We've had 6-year-olds left with younger siblings. That happens quite often," said Katrina Graves, intake supervisor at Prince William County's child protective services. Confirmed "lack of supervision" cases in the county, the largest category of abuse and neglect, doubled in two years to 164 in fiscal 1989.

Project Home Safe regularly gets calls from parents with 7-year-olds in self-care, occasionally from families with 5- and 6-year-olds and recently from a mother who wanted to get tips on leaving her 4-year-old alone, Plantz said. The program advises parents that generally children should not be left unsupervised before they are 12, unless they are particularly mature.

"It only takes an instant for a fire to break out or for someone to break in," she said.

In recent years, there have been several such tragedies.

In the Washington area, two 6-year-olds in Reston died in an afternoon fire after they were left in the care of an 8-year-old in October 1987. In May 1989 an 8-year-old shot and killed his 7-year-old sister while alone at their grandmother's Franconia town house after school.

And in September, an 8-year-old Fairfax County girl was beaten to death shortly after she got home from school. A man who formerly lived with the girl's family has been charged with her murder.

Police say they often hear about young children left to care for themselves only after they have been hurt and neighbors call looking for help for them.

Parents often feel guilty about leaving their children, but rationalize that their children are particularly mature, said Patricia Watters, chief of child welfare services for Montgomery County.

"We'll go out there and the child will let us in, and when the parents come home we're sitting on the couch," Watters said. "Then the parents will say: 'My child is very mature. He would never let a stranger in,' " though the social worker had never met the child before.

"Parents go through a lot of heartaches, but in light of economic realities that is what they do," Watters said.

A Washington Post poll last year found that 15 percent of the children ages 8 to 13 in the Washington area are primarily responsible for caring for themselves after school hours while their parents work.

Maryland requires that all children under 8 be supervised by someone at least 13 years old. District and Virginia laws are broadly written, so that criminal neglect cases are determined on a case-by-case basis rather than with a minimum age requirement.

Several local jurisdictions have their own guidelines for self-care, with 10 or 12 being the most common age used for leaving a child for more than a short time.

In the session at Bunker Hill Elementary, D.C. Hotline worker Simpson told the second- and third-graders about the best way to carry a key, what to do if they lose the key and are locked out of the house, what to do if the house has been broken into and how to handle phone calls and strangers at the door. She had them play-act some of the scenarios with her on play phones.

They also get stickers with the number of PhoneFriend, where children can call between 3 and 7 p.m. if they are feeling lonely or scared or just want to talk.

One Bunker Hill student told Simpson she doesn't get scared: "I feel safe when my mother and father leave me, because I have a dog at home."

Bettye Taylor, who has been a teacher and counselor at Bunker Hill for 27 years, worries about what the latchkey phenomenon will do to this era of children.

"I know how important it was to my children to have someone at home with them. It breaks my heart to see these little babies with keys around their necks," Taylor said.

"They don't know nursery rhymes. TV is raising the kids."

There is no specific law on the age at which children can be left unsupervised. Situations are assessed on a case-by-case basis. MARYLAND

By law, children under 8 must be supervised by a reliable person who is at least 13 years old. Local Guidelines

Anne Arundel County: Children under age 12 must be supervised.

Howard County: Children under 12 must be supervised. Children 8 or older may be supervised by a child who is at least 13.

Montgomery County: Children under age 12 must be supervised.

Prince George's County: Children age 12 and under must be supervised. Children 8 or older may be supervised by a child who is at least 13. VIRGINIA

There is no specific law on the age at which children can be left unsupervised. Situations are assessed on a case-by-case basis. Local Guidelines

Alexandria: There are no specific age limits. Situations are dealt with on a case-by-case basis.

Arlington County: Children under 6 may not be left unsupervised. Children 7 to 9 may not be left alone for more than 1 1/2 hours and they cannot be responsible for younger siblings. Children 10 and older may be left alone if there are no emotional or medical problems and the child is comfortable with being alone. Children 11 or 12 years old may supervise siblings over the age of 4 if no one involved has emotional or medical problems. Teenagers 13 to 17 may supervise younger children.

Fairfax County: Children under 6 may not be left unsupervised. Those 7 to 9 may not be left unsupervised more than 1 1/2 hours and may not be left alone on a regular basis. Children 10 or older may be left alone if they have no emotional or medical problems, but those 15 years old or younger may not be left alone overnight. A 12-year-old may supervise children over 4. A 14-year-old may supervise infants and children.

Loudoun County: Children under 6 may not be left alone. Children under 12 should not babysit. With children over 12, age and maturity should be considered before they are left alone.

Prince William County: All children should be trained in self-care. Children 9 to 11 should not be left alone for more than 1 1/2 hours in daylight hours. Those 12 to 15 years old may be left alone all day. Children 16 to 17 years old may be left alone overnight. Children 12 to 13 may babysit for up to four hours. Children 14 to 15 may babysit for more than four hours, but not overnight.

SOURCES: Local jurisdictions, COPE.