I would suggest lots of books for them to look through during the car ride. That kept my niece busy for quite a while when she was younger ~ and actually still does now as she gets older. Maybe start them off in pajamas too so they can sleep comfortably in their car seats.
The biggest advice I have gleaned from my one sister-in-law is to be sure to keep as close to a regular schedule as you can as far as naps go. They would leave the parks each afternoon, have a little time in the pool, then let my niece and Mommy nap while my brother did his work-out. Then they would return fresh and rested for dinner and the evening fireworks, shows, and attractions. That kept my niece (and sister-in-law) from having melt-downs from being too tired and gave my brother a chance to keep up with his sports training which kept him happy too
The other thing I would suggest from having read it on here is to go ahead and buy some Disney related stuff ahead of time, pack it in your luggage and dole it out during the vacation. Your children are too young to care if their Disney t-shirt is from the clearance section of the Disney catalog or the "latest in moon fashion" from the park!
