JWs love precocious children. If you can get a very young child to give a comment in a meeting, or read a Bible verse, everyone gets the warm fuzzies. Later, they all comment admiringly about how those parents are raising fine young JW children. It's a feel-good moment. If they can get the children to offer a pamphlet or even give a short sermon, both the JWs and the person at the door get to share an amusing moment, and JWs feel as if this is an awesome way to witness for Jehovah.
My guess is that the ones that are most precocious when they are young are the ones who leave first, because they rarely have any understanding of the Bible or even of JW beliefs. To them, it's all a show, and once people stop admiring them the buzz is gone and they realize that there's nothing underneath the fluff. You can teach children, or you can use them as showpieces. The latter does not work out in the long run.