We run a rewards program for all our kids that we call Banana Bucks where the kids get a banana stamp in their "savings book" for several things like just showing up, bringing a bible, knowing the memory verse etc. Every quarter we open the banana bucks store where they can redeem their stamps for small toys or save them for a big prize (our current big prize is a cotton candy machine.)
We have two very troubled children that attend our program and we've extended the banana bucks program a bit for them. We targeted one specific area for each of them (not hitting for one child, no yelling/screaming at other children for the other) and we give them extra stamps at the end of the morning if they've done a good job at trying to curb their behavior.
So far we've found the postive rewards have worked well with all our kids.
By the way, our store is run by some of our older parish members. It's been a great way to include them in children's ministry and they encourage the kids each week in church to keep earning points, even coaching a few at coffee hour on their memory verse.