Caught my kids kissing!!!!!

I just caught my 6 year old daughter and 8 year old step son kissing. I mean like his hand on her waist her hands on his shoulders. Not a cute sibling innocent kiss. His dad hasn't gotten home from work and I'm very uncomfortable. I know a few months ago, my daughter told me they had kissed but to please not tell anyone and that she promises to never kiss him again. I don't know how to address it or what to say. When I asked him what they were doing , he simply said playing tree house and she was bawling her eyes out yelling "we weren't kissing mom!" So now, I don't know how to address it. I told him we were all gonna have a talk when his dad got home and now he's scared shitless. Please help me. 😔 what would u say or do?