We don't happen to own a car, but that hasn't stopped us from buying several of these car litter bags for family members.
Designed and handmade by Allyson Hill, the bags can hang from the back of your headrest or from your stick shift if you have one. Allyson has lots of great fabrics to choose from many of which are kid-themed. Each sells for $19. See the current selection in her Etsy shop.