we do 14 hours with 3 kids, 12, 6, and 1 1/2....dvd, mp3 gameboy, color books, magazines, quizz books, no problems, Get good car chargers/power cords (
walmart sells one that adapts to any size for 15 bucks), lots of batterys, headphones, a dual headphone jack....
a little tip...i make custom cds for each of us, we play them during the trip for the child who is currently in favor for good behavior.......i include funny, unexpected songs for the kids so it's a surprise, also i take the disney vacation planning dvd for them to watch along the way and make their plans.
also, i copy the maps for them and let them plan their own routes in the parks they have lots of fun doing that.........