With children's ministry, I can usually evaluate the rate at which our kids are grasping what we are teaching by the enthusiasm of the volunteers who are leading their classes. I have never had a volunteer come running out of a classroom on Sunday morning after service saying, "I am so excited, I had five kids who wouldn't sit still or pay attention." In fact, the opposite is usually the case. They will come to me and let me know how awesome it is that they get to serve, and that today in their class they had 5 kids invite Jesus into their heart.
I try to evaluate my leadership team on a pretty regular basis. If they are excited to come to class each week, it usually means that they are enjoying what they have the opportunity to do. Happy teachers are almost always better teachers, and better teachers equal kids who are learning in a fun and exciting environment.