When Trading Places Works Well for Families : Exchanging homes can slash vacation expenses and open doors to cross-cultural experiences.

At first, everyone thought Susan and Marty Weintraub were nuts: They had invited perfect strangers with four young children to live in their suburban Baltimore home for three weeks while they and their kids moved into that family’s London townhouse.

“Everybody thought we were taking a big chance to leave our house like that,” said Susan Weintraub, a 37-year-old school librarian. “But we didn’t have a single qualm.” The Weintraubs are having the last laugh. Not only did they save thousands of dollars, they had a fabulous stay in England complete with the kind of cultural interaction parents wish for when they take their children to a new place. Their sons were even introduced to cricket by children they met in their London neighborhood, and in return, taught their new-found British friends how to play baseball.

Whenever the three Weintraub children tired of touring castles and museums, they could go home to a big, child-oriented house complete with Nintendo and Barbie dolls and a whole block full of children eager to play with new American friends.

The British family who lived in the Weintraubs’ house was just as happy, the Weintraubs report, enjoying the swimming pool and friendly neighborhood, as well as all of the nearby historic sites. The two families saved even more money by exchanging the use of their station wagons.

Currently, the Weintraubs are busy studying French maps. They plan to spend three weeks this summer in an architect’s duplex in the heart of Paris’ 16th arrondissement , a very tony neighborhood. At the same time, the architect’s family will take up residence in the Weintraub home in Owings Mills, Md.

“As more people want to keep traveling with their kids, house swapping is getting more and more popular,” said California author Valerie Wolf Deutsch, who describes the practice in a new book she co-authored, “The Best Bargain Family Vacations in the U.S.A.” (St. Martin’s Press).

“We met so many nice people. We were wined and dined all over the place and there wasn’t anything in it for them except to be friendly,” added Evelyn Clair, a San Francisco teacher whose family also are veteran home swappers.

The exchanges aren’t always to exotic places. The Clairs have exchanged with Californians who live in Laguna Beach, and once spent a memorable vacation in Sheboygan, Wis. “I wanted my kids to experience a really different part of the country,” Clair explained.

Sometimes the swaps are done informally. Last summer, Jane and Carl Smith swapped their home in the Chicago suburb of Evanston, complete with car and cleaning lady, with that of a former neighbor who now lives in London.

“Ask friends who live where you want to go if they know anyone who wants to swap houses,” Deutsch suggested. “You can even ask them to put a note up at a local university there.” An ad in a local paper could produce results. Smith notes that many people who now live in faraway places want to return for visits--and still subscribe to the hometown weekly.

Or you can go the more formal route, as did the Weintraubs and the Clairs. The oldest and largest exchange companies in the United States are the Vacation Exchange Club (P.O. Box 650, Key West, Fla. 33041; 800-638-3841) and Intervac U.S./International Home Exchange (P.O. Box 590504, San Francisco 94159; 800-756-HOME).

Here is how those agencies work: In return for a relatively modest fee (roughly $60 a year), your home is listed with thousands of others around the world. Then it’s up to you to make the match, contacting people who live where you want to go. Plan on three weeks for an exchange in Europe. Any length of time--from weekends to weeks--goes for exchanges with people in this country.

Those who live on the East and West coasts, the agencies say, typically have the most choice.

But remember, the agencies don’t check out the homes, and the process takes work and time. Weintraub, for one, wrote some 40 letters before she connected with the Paris architect in whose home she will soon stay.

It is important to be flexible about dates and to interview each other through letters and phone calls, spelling out the ground rules, finding out whether there’s a washing machine or a nearby playground. It helps if kids in both households are of similar age. That way the necessary baby equipment, Legos or video games will be waiting at the other end.