Even if the elders have no legal duty to report an accusation to the authorities, the branch office of Jehovah’s Witnesses will instruct the elders to report the matter if a minor is still in danger of abuse or there is some other valid reason.
So basically it should only be reported if there is a danger of the child being molested again. Don't bother if it's just a one-off. Or there's been lots of occasions but it's not expected to happen again. Bastards.