I have my cousin who I went to visit, he has 3 kids of which he has 2 girls..The little one is 3 yrs old...?

she is still getting potty trained, and today she was playing with her poop. She obviously had to get cleaned up in the shower, so that's what he did, he says that while he was showering her he accidentally smacked her face with the shower head where she had poop...now she has absolutely no other markings in her body and I freaked out when I showed up to see my niece that way, but he was the one that called me to tell me what was happening and that he knew how bad this was going to look to anyone else...his girlfriend was at work at the moment and I went ahead and called her for her to know...she's freaking out trying to report him to the police, but he would never hurt his kids this way, I believe that it was an accident or else he would have made up a lie as to how she got the bruises. How do we go about this, has anyone ever been in this situation? I'm just a mediator here and I am trying to do the best thing possible for all of them...thank you all, and please remember this IS a sensitive issue and I am just trying to get some enlightenment.