I am a Mom of 3 sons who are now 20-14 yrs old. We did many drives to WDW from NJ when they were as young as 3, 5 and 9... We did it just as the portable systems were available and we travelled with our oldest to Florida when there were none.
I am lucky they were great sleepers in the car. My oldest was the reader, so books were his thing.....for the younger travel size games, coloring books, books to read and some fun music on the radio helped.
I still play the license plate game with them...they.learned their states, later we worked on the capitals.
As time went on we had the Leap Frogs etc. We have found making sure they stopped enough to stretch and go to the bathroom helped. We always have a nice long leisurely lunch ....our stop is Cracker Barrel, since we have none by us. We do not mind if it busy, they sit in the rocking chairs, play checkers and look around the store.
Today, with car entertainment systems and iPads....traveling with children can be much easier. But we still love the simple stuff reading the roadway signs ...how many miles to South of the Border.... And hey Mom ...I just saw An Alaska plate.
Good luck with your trip and planning.