When traveling in the car or on an airplane, a great entertainer reusable stickers. Your children can stick and re-stick them on the windows of the car or airplane. Also, lollipops are good for earaches during flying.
Take lots of children's stories and music on tapes, plenty of cheap toys, and new books.
Wrap individual goldfish, gummy bears, and (a small amount) of M&M's individually in aluminum foil. Small children will delight in unwrapping them to see what's inside. This is a great tip for plane rides.
Sounds obvious, but ... Make sure you have your children go to the bathroom before you leave the airport - and have something for them to do so you have a nice flight there and back.