Sitting still and quiet for hours on end in a cramped space is tough, even for adults. Make sure your children know before you leave that good car behavior will be rewarded. Offer extra money to spend at the gift shop, a choice of desserts when you stop for dinner, or one-on-one time with each child during the trip. If you’re visiting relatives, give out “travel tickets” kids can cash in for treats or favors when you return home.