As a father many times, and having made dozens and dozens of long interstate drives with kids, I can tell you that if they sleep in the car, then they’re up at the destination. You have to decide how much peace and quiet in the car is worth the hassles at the end. Generally, if they’ve been up the whole trip, and there’s an hour to go, I hope they stay up in the car so they’ll sleep when I stop.
It is not worth the one hour of sleep in the car for the three hours of awake at the end.
Three generation family vacations are inadvisable.