We took a road trip a couple years ago. In preparation I found a great website: http://www.momsminivan.com/index.html
It has fabulous ideas! I hope you find something useful.
One thing I remember my mother doing when we were kids and made that drive at least once a year, was to take a piece of chalk and divide the front tire into sections with numbers or names in each section/ Then each time the car stopped (for gas or potty break, etc) the number or name told us what we would do on the next leg of the trip. For example, if my name (or initials) were on the top, I would get to choose the music we listened to or the snacks we ate, or make whatever choice there was to make (we didn't have DVD's, but that could be a choice, too). Or, if you put numbers then the numbers could represent the choices, ie. 1- movie, 2- CD's, 3- play a family game. Or each number could represent a different treat. Maybe the kids could earn extra money to spend at Disneyland by getting along well or being in charge of something specific in the car. One could hand out drinks, one could be in charge of the dvd's, etc. Ok, so I just looked at the ages of your kids, maybe they aren't old enough for all that, but you get the idea.
Books on CD are a great way to pass time, too.
Another idea...my parents used to leave at bedtime and we would pretty much sleep the whole way. The trip always seemed short that way. We would go to sleep and when we woke up we were there!
Have agreat trip!