Place the child in a cage with a cuddly animal for several days. Dark, and all alone except for the animal they will bond with it. Then several days later during a ceremony, they will remove the child and animal from the cage, kill the animal and force the child to eat its heart. That's damage the child will never recover from...
These people doing the bidding of the evil, need to die to save future victims.
Further reading suggestion: Dr. Norman Doidge has a few books about brain plasticity. Not specifically about this, but rather a series of anecdotal cases and researchers who document unorthodox cures to brain injuries and trauma. Relating to suppressed memory, his work suggests that when the input is too much to be absorbed immediately, the information is suppressed; releasing it involves replaying the memories and emotionally processing them as if lived in slow motion, when one is better able to process them. A fascinating topic in itself.