There are 2 ways you can do this.
1. Start the trip after dinner. Get the vehicle loaded in the afternoon except for an ice chest with juice and some snack foods. Then sit down to a nice family dinner and enjoy. Get kiddo all fixed into the car seat and head out of town.
Stop a couple of hours later for diaper change, run around time, and an evening snack. Put kiddo in jammies/brush teeth, etc.... and get them settled in with a movie if you have a DVD player. They should have a full tummy and the vibrations of the vehicle should put them to sleep. Even if they don't go to sleep the movie should keep them occupied.
If they're still awake then stop again after the movie is over. Letting them run around and run off any energy is one of those things you have to decide about. If they get wound up by playing and running around then you might not want to stop except to change the diaper.
I'd imagine they'll be asleep at the end of the movie and I'd be able to drive straight through. That's the ideal of leaving after dinner.
2. Get up and start in the morning. Eat a good breakfast and then load up. Plan on stopping every couple of hours so the little one can get up and be on the move. We always stop at McDonald's, public parks, and other play areas so the kids can run amok for at least 20 minutes. They are going to be a little sore from sitting so long. Think about what that seat feels like on their hiney, your seat is padded and comfy, theirs is hard plastic with a thin sheet of foam on it.
Have snacks in the ice chest that are already cut to size. I didn't give the kids snacks unless one of us was sitting beside them at this age. I had one that got choked pretty easily and it was super scary trying to pull off the highway and flipping them upside down.