Being cooped up for hours on end isn't fun for anyone, so be sure to take regular rest breaks. While you're stopped, help your children burn off some energy.
Pack some small athletic gear in your car. Small, squishy footballs and Frisbees don't take up a lot of space, and they can get the entire family involved and the blood circulating.
If you just don't have room for sports gear, make up some quick, simple outdoor games. Create races where everyone has to run from tree to tree in different ways: Run to the first tree, hop to the second, jump to the third and run back to start.
Getting everyone moving will make your driver more alert, and it will help calm down your children, maybe so much that they'll nap for an hour. That will definitely give you some silence.