My grandmother made all of us "car quilts". She didn't put pockets on them, I was afraid some candy would get stuck in them and make a real mess. I did collect patches from our travels and sew them on the back. I suggest a matching small hanging with pockets to put on the back of the seat in front of the kids. You could put pencils, paper, books, snacks etc in it and it would be easily removed.
Here is a link to a pattern, but you should be able to figure one out:
http://www.patternsonly.com/car-seat...rn-p-3410.html