When I was visiting The Grove Community Church in Riverside, California, I saw a system that they used to help motivate kids to listen & participate. They used ping pong balls to give to the kids when they brought their Bible, or a friend, etc. At the end of the service, the kids could put the ping pong balls that they earned into their display box. The group who filled their box first was rewarded with a pizza party. I liked that the reward system was visual for the kids, and they worked together to achieve a goal.