Find something that the children enjoy holding, like a particular stuffed animal that could become your “club mascot.” Allow the child who is the best behaved to hold the object during the next club or time segment. (Bible lesson, memory verse, missions story, etc.)
This might work best with younger children, so as a variation that might work with either, try a locked box (or that treasure chest I mentioned a couple of weeks ago) with a special prize in it. Before you begin, choose someone at random to hold the box. At the end of class or club decide on whether or not that child behaved well enough to get the key to open the box and receive the prize.
You could decide, if you wished, to simply choose a quiet seat winner each week to come and receive the key, but it might be more fun to have someone hold the box during the teaching time. It would also be a reminder to that child to behave.
If the prize is not earned (and make sure that they understand the prize is earned and not deserved) it would remain for next week.