In today's society of many single-parent families and two-career couples, baby sitters are very important, and especially so when one or both parents must go out of town on a business trip.

Who will take care of the children?

A new firm, Sitters Unlimited, has been established locally to cater to traveling parents. It is part of what is foreseen as a "coast-to-coast" network of similar firms, supplying sitters in cities of major vacation and business travel.

Here in Washington, Sitters Unlimited offers two major child-care services to travelers:

* It provides sitters for out-of-town visitors who bring their children with them. They sit with the youngsters in the hotel or will go touring with them if special arrangements are made.

* It provides overnight sitters for Washington-area parents who must travel out of town. The sitter will stay in the child's home for a day, a week or even longer.

Recently, the firm added a third, and unusual, child-care service. It operated a day-care center for several days during a convention of the American Dietetic Association of Chicago, held at the Convention Center here in October. Delegates, including several nursing mothers, were able to leave their children during convention business meetings.

The firm also can provide temporary sitters for the elderly, for pets or simply for housesitting.

Sitters Unlimited is a franchise business, originating in the Los Angeles area, where these sitting services also are available to travelers (principally in Orange County and Long Beach). Within the next few weeks, franchises are scheduled to be operating in Las Vegas and Dallas. And founder Nancy Ann Van Wie of Laguna Hills, Calif., who once ran a federally funded pre-school program, expects in the next three years to open branches "in every major city."

The local franchise is owned by Nancy Richards, an Alexandria educator whose background has been work with handicapped adults. Following Van Wie's guidelines, Richards says she makes a special effort to hire sitters who have professional experience in education, nursing or social work, including students (minimum age 21) in these fields. Many on her on-call roster of about 60 sitters are part-time teachers interested in adding to their incomes.

Obviously, many parents are hesitant about leaving their homes and children with strangers. To ease their fears, Sitters Unlimited makes two advance visits to the home:

The first is by one of Richards' directors, who get answers to a long list of questions, ranging from the child's eating habits to the location of the main water valve, in case a pipe bursts. The second is by the baby sitter, so the family and the sitter can become familiar with each other.

What she has found, says Richards, is that many wives who previously could not accompany their husbands on business trips because of the children now feel confident about going. For them, "It's a dream come true."

The cost of in-home child care varies according to the number of children and their ages. The basic overnight fee is $50 a day for one school-age child 6 and above or $320 a week. For a child 5 and under not in school, the rate is $55 a day or $350 a week. Holidays are extra.

The hourly sitting rate for childen is $5, with a minimum of four hours or $20. (For companion care for the elderly, fees are $7 hourly, $70 overnight and $450 weekly.)

"We're seeing a lot of single parents, and not just mothers, who are bringing their children with them to Washington on business trips," says Rissa Woodley, one of Richards' directors.

Recently Woodley got a frantic call from a father who had brought his 8-year-old son to a convention and didn't want to leave him alone in their downtown hotel. The hotel referred him to Sitters Unlimited, and Woodley spent a full day with the boy doing what he wanted most: riding Metro, including stops to watch the airplanes at National Airport and a picnic lunch in the park.

Once she spent a week sitting two dogs and three cats in their master's home. They were, she says, a well-mannered lot, trailing her about the house in orderly file. "I felt like a Pied Piper." Live-in pet care is $30 a day or $190 a week. Drop-in pet care is $18 daily and $115 weekly.

As a mother of a now-grown son, she personally has seen the need for qualified sitters to be available to travelers. She recalls once arranging for a sitter through her hotel desk. They sent an off-duty maid who didn't speak English. Several Washington hotels now refer guests to Sitters Unlimited.

Richards and Woodley make it clear that Sitters Unlimited is interested only in temporary sitting assignments. They say they can better maintain the caliber of sitters if the sitters are working professionals in education looking only for occasional extra income.

To put baby sitting on a business basis, Sitters Unlimited has established certain rules for both sitters and clients. Sitters, for example, are prohibited from smoking, consuming alcoholic beverages, making personal phone calls or having guests. Clients must make a deposit in certain circumstances, and cancellation fees are charged.

When she originated Sitters Unlimited in 1979, says Van Wie, her only intention was to open "a service for working mothers." She quickly discovered, however, "a greater need by the traveling parent."

For more information about Sitters Unlimited in the Washington area: 823-0888 (7 a.m. to 6 p.m.). For the national office of Sitters Unlimited in Laguna Hills: (714) 380-9040. In the Los Angeles area, several listings for Sitters Unlimited in Long Beach and communities in Orange County are in the phone book, area code 714.

* RENT-A-PARKA: Yet another new idea, this time in the ski industry -- rental ski parkas and trousers (bib overalls).

Like rental skis, boots and poles, they come in handy when:

* You've never skied before, and you don't want to invest a lot of money in an activity you may never repeat.

* The airline lost your luggage and you need warm clothing until it shows up.

* You ski so infrequently that you don't want to buy something that will go out of style before you wear it again.

* You are on a winter business trip to Denver, toting only executive wear, and suddenly you have the day free. You can rent almost everything you need, except gloves and a hat.

Gerry, a U.S. manufacturer of down skiwear, has introduced its "Down Demos" skiwear rental program at 300 ski shops and ski resorts nationwide this year after a trial test last year.

Locally, the Ski Chalet (2704 Columbia Pike in Arlington) is offering the outfits at a daily rate of $14.75 for adults and $9.75 for children's sizes. The navy blue or red parkas and pants are "brand new," says Don Owens, rental shop manager, and are dry-cleaned after each use. He has about 80 sets in all sizes available.

The Gerry parka and pants combination customarily sells for about $224.

Near Denver, where the ski season begins this month, Gerry rental clothing is available at the Vail Ski Shop, (303) 476-5337, and the Ski Depot at Winter Park, (303) 726-8055, at comparable rates.

Vail is about 100 miles west of Denver and Winter Park is 67 miles west, both convenient for a day's round trip from the city. Once there, in a matter of minutes you can strip off the pin stripes and head for the slopes in this year's fashions.

Gerry offers a free "Beginner's Guide to Skiwear." It over-promotes the firm's product but has good tips, too. For a copy: Gerry, Box 3431, Merchandise Mart, Chicago, Ill. 60654.

* UGANDA WARNING: Due to "poor security conditions," all but "the most essential travel" to the east African nation of Uganda should be avoided, advises the State Department.

"Casual overland travel is particularly discouraged," officials say, but business travelers who exercise caution "can continue to visit Uganda in relative safety."

However, the State Department travel advisory warns that visitors should "make firm arrangements in advance with sponsors who have sufficient experience and resources to cope with the security situation."

Overland travelers, it says, "will encounter official and, occasionally, unofficial roadblocks and should exercise extreme caution if stopped." Certain installations and buildings are considered sensitive, and travelers approaching them "may be stopped and detained without warning."

Americans going to Uganda should make their whereabouts known to the U.S. Embassy, located in the British High Commission Building.

* ANCIENT MEXICO: An unusual 17-day art-oriented tour to Mexico's Mayan ruins, colonial towns and Indian villages, Jan. 21 to Feb. 6, will be led by Tracy Atkinson, director of the Wadsworth Atheneum, Hartford's renowned museum.

The one-of-a-kind excursion, departing New York, will begin in the Indian city of Oaxaca, south of Mexico City, famous for its market and churches. From the southern city of Tuxtla Gutie'rrez, travelers will go by bus into the Chiapas mountains to the former colonial capital of San Cristo'bal de las Casas, visiting Chamulan Indian villages along the way.

From the mountains, the descent is to Palenque, a classical Mayan site in a tropical rain forest, and the tour continues on to the Yucatan city of Campeche on the Gulf of Mexico. This part of the itinerary is "off the ordinary " tourist route, says Atkinson, although he assures that accommodations "are uniformly quite fine." The final stop is Me'rida, where day excursions are planned to major Mayan ruins.

Cost of the tour is $1,950 per person (double occupancy), which includes transportation, lodging, most meals and a $150 tax deductible contribution to the museum. Open to a maximum of 25 participants.

For more information: Wadsworth Atheneum Art Tours, 600 Main St., Hartford, Conn. 06103, (203) 278-2670 or (203) 674-1114.