It never occurred to me to get either of my kids a gift--the gift of me letting them drive one of my cars seemed sufficient!
However, I can see how you might want to give them a little "token."
How about a gas card, a pass to your local car wash (if they have those in your area), a travel mug, a sunglass visor clip. Just some little "car" stuff that you could all put together in a gift basket (or stuff it in the travel mug). It really just depends on the things your child likes.