Caught my 5yo son and his 5yo girl friend being sexual.

I need some advice, recently we caught our 5yo son and his 5yo female friend being sexual in his room.It was just a play date and these two are best friends from Kinder. She came over for a visit, her mom and my wife and I were chatting in the lounge room while the kids played.We first noticed inappropriate signs when the she took him into the cubby house and covered it with a blanket, and then it was quiet and we could hear kissing noises. She'd poke her head out being cheeky and saying she's having a baby.Her mother explained her daughter knows about the "birds and the bees" . Whereas my son doesn't know anything about that stuff yet.Later the kids went into my sons room, us parents were having a good chat and realised it was a bit quiet after a while. My wife went to the door and heard her say "just get it out". My wife went into the room and was stunned! They were laying in the bed together, his pants were down and she was playing with his penis.She immediately stopped them, we later sat down with our son and tried to have a talk. He won't tell us anything, we haven't been approaching it in a bad way or anything, we know it's not their fault, she seems to be wanting to explore it a lot. But I'm sure a lot more went on because they were in there for a while, we just thought they were playing with their toys.Every time they catch up now, she keeps trying to explore with him, mostly just kissing and holding hands. We don't let them be alone without supervision anymore.We don't know what to do, I feel like he's waaaay too young for any of this, he calls her his girlfiend and in a serious way. I feel like he shouldn't have to worry about this stuff for another 10 years. I worry it will can effect his mental health, especially if he has to go through break ups already. He's 5!Anyone else been through anything like this, or have any advice? via /r/Parenting https://ift.tt/2FqLtZg