This is an interesting idea!
it could certainly work, but be careful it doesn't turn into a works righteousness competitive sort of thing.
There is a good chapter in Show Them Jesus by Jack Klumpenhower about that problem. ("the Grapes that Taught Good News")
from page 158:
"I want to teach them the good news, which tells us (1) the best rewards are not material but spiritual and (2) these are never earned or deserved by our religious behavior."
i highly recommend this book. It will help you look at Sunday school differently.