what do we do???

I need help. I don't know what is going on. I take care of my stepdaughter during the days. She is 6. Her and my son were playing in my sons room. My son is 4. They had been quiet for about 5 minutes. So, I decided to go check on them I watched them through a crack in the door. I saw my stepdaughter telling my son, " OK now I'm going to be (her moms name), you go in the closet and come out." My son went into the closet and came out. My stepdaughter said, "Hi Im ( her moms name)" then my son said, "hi". Then my stepdaughter said, "shhh. the kids are asleep". Then They were out of my view. so I opened the door and asked what they were playing? My stepdaughter ran and covered her face. My son said, "oh we're playing ( my stepdaughters moms name)". But what I saw shooked me. My stepdaughter was hugging my son. and pretending , ( I hope Pretending to) make out with my son. She was sitting on his lap facing him and trying to open his mouth to her mouth. I don't know what was going on before I walked in. They were only out of my sight for 5 minutes while in the room. Then they were out of my sight for about 10-20 seconds before I opened the door.
I have a feeling her mom is having sex with her boyfriend while she thinks her kids are asleep. I get that from the way my stepdaughter spoke and acted. Am I over reacting or do I have valid concerns? Please give me some advice. I live in CA.