Help!! My son did something that blows my mind and I need advice!!

QUEEN

okay so about 2 weeks ago my son and my daughter my son is 5 years old and my daughter 3 years old. They were playing on the trampoline and we looked outside the check on them and my son is laying on top of my daughter with his pants down rubbing his weiner on her belly. He has not done it since in what we had told him if that is your private part nobody should see it and you should not see anybody else's. That's our special place. He has not done anything like that since. And I have no idea where you would even learn that from because I don't watch Dirty shows all I watches cartoons and I'm just really confused and need some advice. Please help