Go to the dollar store and buy a bunch of cheap make-up, toys, and anything that looks like fun for kids (age-appropriate items) and if they don't whine, argue, complain, or attempt to hurt each other - they get prizes every 50 miles or every 45 minutes.
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. (Make sure to keep a trash bag next to him, though).
When taking long road trips with younger children (6 and younger) have them wear diapers or pull-ups (or for older kids, goodnites) while riding in the car. This has saved us from many accidents. If they refuse to wear them, keep a couple in the glove box and have them put the diaper on when they need to go.
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. Good Luck!!!!