We drove down a few years ago and our girls were about the same age as your children. I ordered (or you could even make one) something to hang on the back of the front seats to hold all sorts of fun activities (they knew about the trip, but the activity bags were a surprise). I bought most of the items at the $ store. Crayons, coloring books, magnet activities/games, pipe cleaners, small craft kits, paper dolls, etc. (things you know they will like). I bought a medium dry erase board for each of them. They used it to draw, but it also came in handy for a "tray".
The portable DVD player or laptop was beneficial for playing movies. Our library allows movie check outs for 2 weeks at a time. They are free and no late fees! If not, Redbox is a great choice. If they have any hand held video games, those are entertaining.
Search the web for scavenger hunt pages and print them out for the ride.