The two witness rule is used by the JWs only as part of their own internal judicial system, to establish sin for disciplinary action of the one committing the sin but has a number of negative consequences.
Of course child sexual abuse is a crime and not a sin , so Elders who are informed of an allegation of sexual abuse against one of the children in the congregation regardless of whether it was committed by a fellow member or a non member , it should have been reported to the appropiate authorities when it came to their notice. regardless of whether a mandatory rule of law applied or not.
It was a moral responsability on their part which they failed to do.