The Golden Child is supposed to be "invincible," but there is some evidence that s/he is susceptible to certain Island Events (and shouldn't be). My leading theory for the continual drain on the child's health that you observed is that one of the Island Events made the GC sick, which isn't detectable because the GC's status always says "Golden Child." Once the child's health reached the 50% point, the other Island Event caused the loss of the remaining health. This should all be fixed when the v1.1 update is released, but if this happens to anyone else, please try healing the GC and report what happens!