Don't leave the ignition key in the car, not even for a few seconds. Don't allow arms or heads outside the windows. Don't worry about kids having to sleep lying down. They can sleep sitting up, but bring a pillow for comfort. Don't carry unsecured cargo.
Loose objects become lethal missiles in a collision.
Put loose items in the trunk. Don't settle feuds while driving. Pull off the road at the first safe opportunity and then lay down the law. Don't drive without using a safety belt yourself. Set an example for safe driving and tell the kids it's the grown-up thing to do.