Plan your trip with enough time to only drive a maximum of 6 hours per day. Plan longer stop rather than a lot of short ones. If you take a longger break and let the kids get out it will make getting back on the road a lot easier and eliminate a lot of the fussing. Also, plan your start time around you child's normal nap time. This allows the kiddos to sleep for some of the trip.
Take a trip to Family Dollar
Visit your local Family Dollar and grab a few of thing for the kids to do while riding. They have some great mini puzzles, and activity books that are cheap, and will keep the kids entertained for a while. While your there see if you can find a magnetic drawing board. It's fairly cheap and kept