I know that when we were young our parents made us travel for long hours in one stretch, but it is also true that back then we were not tightly tied to a plastic seat with a snug fit!
So what I recommend, for everybody’s sanity, is to stop every two or three hours to let your children move their limbs and get some fresh air. These stops do not have to be very long, even 10 minutes is enough to make the kids happy. Ideally you would stop near a playground but if that is not available then any grassy patch will do: just let them run!
You could even have a picnic, and here comes my second advice: always keep a picnic rug in the car– it will make all these stops more comfortable (and you can place on it the non-running members of the family)!