Lots of great tips here, a lot of which were those we used when traveling long distances with our kids back in the day. There were a few other things that we did that proved particularly useful...
First, my DW and I switched driving duties every two hours or so. That way, neither of us got tired, and each of us had time with the kids. With this practice, we found we could comfortably handle longer travel days.
Second, we made it a practice to get off of the interstate at lunch time. Every small town has a community park, and we'd head to one of these instead of using rest parks for a picnic lunch. The kids could burn off some energy at the playground, and there was the added bonus of getting a glimpse of the local communities. Doesn't add as much time to the trip as you'd think, and breaks up the day nicely when traveling with young kids.
And third, a previous poster had mentioned giving the kids maps. We did this as well, but took it step further by having the kids take turns in the passenger seat next to the driver with the caveat that when there, they had to act as 'navigator' from their map. They kept track of which exits were were approaching, what towns were close by, and even which exit the driver was going to need to take. It's a great way to teach your kids how to read a map which, unfortunately, is becoming somewhat of a lost skill with GPS units and smart phone apps so common.
One last thought... Make the journey part of the experience instead of something to be endured while you get to your destination. It's easy to look at the time spent between points A and B as something to put up with, looking forward only to what you'll do when you get to where you're going. But there's a lot of interesting stuff to see between A and B that can end up making the journey just as fun as the destination.
Enjoy... you'll have a great time!
