That is an especially difficult situation. A few things to consider are:
(1) Ensure the safety of the child, at all costs. If that means placing the child in appropriate agency and/or removing him/her from the current surroundings.
(2) There should be no delay in seeking medical & psychological help for the child. This may not seem easy but has to be done and with as much sensitivity as is possible.
(3) There should be no attempt or permissiveness for covering up the abuse. All necessary and appropriate action should be taken and followed up by the concerned individuals and agencies in this regard, including reporting the matter to the police if required.