I hate to say it, but having a dvd player or a tv/vcr in the car is a complete livesaver! We drove from WA to So CA with a 2 year old and 5 year old and it worked out well. If you don't have the portable dvd player, you can sometimes rent them or, if you're lucky, maybe someone you know has one you could borrow. We did our roadtrip before they were invented but we lugged our 13" tv/vcr with us (we had a mini van so this was a little easier) and got a power inverter for the cigarette lighter. They cost as little as $20-30 and are well worth it.
We were careful not to just let the videos roll continuously but, between lunch in the car (yes, it really will give you a solid hour or two of keeping them busy plus a nap beyond that), new things to look at and do (yes, loading up on stuff from the dollar store is also a must; don't give it out all at once, though; give only one new thing at a time and don't let them know you have more or that will become a new problem in itself), and down times when they are just riding, you should do fine.
For myself and my husband, we didn't do very well driving into the night. He's not a night person at all, so around 9:30 he was pretty much toast. I am, though, so I could drive on until around 2-3 am. Between that and sunrise, though, neither of us could do it. There's something psychological about it for us, I think. It just seems easier to stay alert when there's light out. Keep that in mind when you're planning your trip. It is easier to drive at night when there's less traffic and everything is quiet and the kids are sleeping but, if YOU'RE getting sleepy, too, that's no good! Good luck!