TREKKING through the Roman Colosseum or the streets of New York City with children in tow may not be every parent's idea of the perfect vacation, but family trips can be fun and memorable long after the suitcases have been emptied.
Sanford and Joan Portnoy of Newton, Mass., are veteran family travelers. With their two children, Katie, 10, and David, 8, they have traveled extensively in the United States, Europe, and Canada.
Drawing from their own travel experiences and from talking with other parents about their family trips, the Portnoys have gathered the fruits of their globe-trotting research into a new book, ''How to Take Great Trips With Your Kids'' (Boston: Harvard Common Press, $8.95 paperback).
The book offers practical tips for traveling with children from infants to teen-agers and includes sections on choosing transportation and lodgings, planning an itinerary, packing, cost-cutting techniques, and ideas for games en route.
''We feel strongly that exploring new places together and meeting challenges, such as coping with different languages, bring us together and expand us as a family,'' says Mr. Portnoy, a psychologist who specializes in family issues.
Although accommodating children takes more organization and energy than taking trips alone, the Portnoys have found that traveling with their children adds a special zest and richness to their experiences. ''They are amazing ambassadors,'' says Mr. Portnoy. ''We meet more people because of the children.'' Mrs. Portnoy has found that people are especially kind when they have their children along and will often go out of their way to help if they can.
The Portnoys believe that traveling helps children develop a sense of confidence in their ability to manage in new situations. It may also help them to cultivate a sensitivity to other people and other points of view, giving them a perspective about who they are in relation to the world.
''Your children may not get the same things out of a trip that you will, but they will get a lot,'' the authors write. ''Sometimes the interest you show in the history of a place, or your appreciation of some cultural difference, can turn out to be more significant for your child than you would have imagined.''
To help ensure that a trip will provide something for everyone, the Portnoys include their children in the planning stages and try to achieve a balance of adult and children's interests along the way.
One of the most common trips with children is long-distance car touring. The key to a successful car trip, says Mrs. Portnoy, is to think in advance about activities children can do to pass the time. Some possibilities: projects such as coloring books or puzzles, books to be read aloud or alone, music or storytelling tapes, and cards or other games to play with family members.
A pad of paper and some crayons or pencils are one of the most versatile diversions for any child, the authors note. Many youngsters enjoy compiling a trip diary using post cards, soap wrappers from hotels, nature objects, written impressions, and other memorabilia.
Parents can teach young children to go from one type of activity to another. By the time they're older, they will have established their own car routine.
''We find ourselves playing tons and tons of word games,'' says one mother in Westwood, Mass. The family encourages their children to do their own map-reading and to keep a log of miles and finances - keeping track of daily expenses for tolls, food, and gas.
On a recent trip to Tampa, Fla., from the Boston area, Linda Sjoberg drove alone with her 21/2-year-old daughter, Molly, as far as Charleston, W.Va. There she joined a friend with two young sons, and they continued the trip together.
To keep Molly occupied during the first part of the trip, Mrs. Sjoberg set up the backseat as a play area. Playthings included a lunch box filled with a new set of small toys, some favorite dolls, and a set of books organized in zipper bags. When she got sleepy, Molly snuggled into a quilt and pillows.
In the front seat Mrs. Sjoberg kept a cache of some of her daughter's favorite snack foods - raisins, grapes, and Cheerios. ''I put everything in child-size portions in separate containers so she could serve herself,'' she says. ''That was very helpful.''
On the first leg of the trip, the Sjobergs stopped every hour or so at rest areas to stretch their legs. Later in the trip, after they had joined their friends, the group stopped at a playground and stayed an hour and a half for one of their driving breaks.
''The children loved it,'' says Mrs. Sjoberg. ''They got back in the car and just fell asleep. It was wonderful.''
Breaks for physical activity during trips are a must. If possible, the Portnoys suggest plotting a route that passes a historical site, an interesting factory, or a lake where the family can take a quick dip. Packing all the swim gear in one duffel makes spontaneous swimming stops easier.
''One of the more common mistakes is driving too far in one day or driving too many days in a row,'' says Mr. Portnoy. Five to six hours a day in the car is usually a good limit, with breaks at least every two hours. He also suggests stopping for the day during the afternoon rather than late at night.
''The worst times tend to be at the end of a long driving day, when people are tired and hungry,'' he says. Stopping about 4 o'clock allows everyone to unwind before dinner and eliminates the frantic search for a hotel room and restaurant late at night.
Their rule of thumb is this: When planning your itinerary or a day's activities, think realistically about how much your family can do in a certain block of time, and then plan to do less.
''After all, it's supposed to be a vacation,'' says Mrs. Portnoy. ''It's not supposed to be a contest to see how many places you can get to in two weeks.''