Lets just say that when a child, nebech, is shoved off the derech they go down a very dark journey and go through many different nisyonos. They carry a lot of pain and a lot of baggage with them. Some from the difficulties that sent them on this path and much from the journey itself. Is it any wonder that a child would need some form of therapy to help them on their journey back? Do you think that they just leave their baggage at their last stop and just rise above it?
There is a lot of healing that has to be done when a child returns and realizing that Hashem is on their side is just the beginning. You don’t just dunk in a mikveh and come out purified mind, body and soul.