Pick up some inexpensive items from a dollar store to use for "special awards" that you create while on the trip. These retail stores have other games and sets that are excellent for in-car entertainment.
Hold back some of the "treasures" you have to be used in emergencies only that can cheer someone up quickly. Children can be quite useful in setting up and striking camp.
Children should be assigned to different tasks. What is also known is kids need frequent stops. An RV resort or campground with a swimming pool and playground area is something the children can look forward to.