Resting the camera on the dashboard of the car to keep it relatively stable would be a good approach. Then it's just the car's movement and the cars outside that will create the blur and not camera shake. This will have to be experimental as far as shutter speeds and exposure are concerned. Obviously night time would work best if I am going to record light streaks. The camera pointing at back seat and kids with blur outside?
“Our parents, our children, our spouses, and our friends will continue to press every button we have, until we realize what it is that we don't want to know about ourselves, yet. They will point us to our freedom every time.”~Byron Katie