What would you award points for? That would be my big caveat.
In my experience, it isn't the kid's fault for irregular attendance, or not having a parent who reads them Bible stories at home. And speaking as someone who didn't go to Sunday School very much as a child, a reward system would probably have left me feeling hopelessly behind some of my peers.
The less we try to make Sunday School like public schools the better.
Different subjects, different objectives, different values system.
Certainly there are things we learn from them, but many things I think we are struggling to UN-learn as well!