15 “No single witness may convict another for any error or any sin that he may commit. On the testimony of two witnesses or on the testimony of three witnesses the matter should be established.
Two Witnesses is a valid principle, but not required to be taken literally in every occasion, either in the Bible or Watchtower doctrine.
For instance, Watchtower does not force an adult rape victim to confront the rapist, even though Matthew 18 says that should be the case.
Matthew 18:15,16 "Moreover, if your brother commits a sin, go lay bare his fault between you and him alone.
Deuteronomy 22 shows that if a girl is raped and no one was there to help her, hence no two witnesses to the rape, her rapist could still be found guilty and put to death.
Deuteronomy 22:25-27 “If, however, the man happened to meet the engaged girl in the field and the man overpowered her and lay down with her, the man who lay down with her is to die by himself, and you must do nothing to the girl. The girl has not committed a sin deserving of death. This case is the same as when a man attacks his fellow man and murders him. For he happened to meet her in the field, and the engaged girl screamed, but there was no one to rescue her."
Each of these Scriptures are directed at adults and not children.
Since there is leeway in how to apply this principle, then there is no doubt that when a child's safety is at stake, every precaution should be taken to protect the child from the first indication of danger.