I think my 4 year old nephew tries to have sex with other little girls?

OMG okay so today i went to my aunts house and my nephew who is 4 years old kept calling my friends daughter who is 3 inside the rest room with him she kept saying no but i got distracted for a while next thing i know i see them coming out of the room and the little girl is scared. I also remember another time when another little girl was around him they kept going to a room. Twice when the grown ups noticed they were in there the little girls' shorts were off. I dont know what to do. I was molested as a child so i feel its my job to do something. I dont know if im just thinkimg wrong because of what happend to me. What should i do?!