As someone who has traveled with a child for many years, the key to keeping them well-behaved is to keep them busy. Make sure you pack their favorite toys (those that you can easily travel with). Gameboys are convenient, Game Cubes are not. Bring crayons and paper, books, etc. And if you ask to be seated near other families, there is a good chance your kids will bond with other kids around their age. That way each set of parents can take turns taking a break from watching them.
Bring medications just in case, such as cough medicine. You never know when they might wake up with a sneezing or coughing fit. There is nothing more annoying to other passengers than a kid constantly coughing at 2:30 AM. Trust me, I know the dirty looks!
And the busier they are during the daytime, the better chance they will sleep better at night.
Have a nice trip!