TRAVEL BY TRAIN
While train travel isn't common or even feasible in much of the U.S., you can zip across Europe easily and cheaply due to their network of high-speed trains.
With the kids in tow, train travel is especially convenient. It costs a fraction of airfare, and you don't have to deal with airport security drama, either.
DON'T OVER PLAN
While you might feel tempted to pack your days and nights with all the sightseeing you can fit in, having the kids along offers the perfect excuse to slow things down.
Instead of packing your trip with tours and attractions, plan one big "thing" to do each day and go with the flow the rest of the time.
Not only will this help your kids avoid sightseeing burnout, but it will save you money, too. The last thing you want to do is pay (or overpay) for attractions and events your kids are too crabby or overtired to enjoy.
TAKE ADVANTAGE OF FREE ACTIVITIES
Especially in Europe's big cities, you'll find plenty of free and cheap things to do. Look for city parks and little touristy attractions that will keep your kids entertained for hours for very little expense.
A small city carnival, a city parade or even a busy waterfront can give kids a place to play and be kids while the parents relax and take in the view.