What do I do??

So I have a step daughter that is 4 and my husband and I have had joint custody of her with her Mom since she was 1. So tonight i walked in her room to check on her while she was playing and I think I caught her humping a stuffed animal. 😳 I’m mortified and I am at a loss of what to do. I just asked her what she was doing and she said nothing and I said okay well don’t do that and she said okay. Do I tell my husband/her dad?? I’m sure it will mortify him also. Or do I just leave it at that?? Children don’t just come up with that on their own right? She had to have seen it or something at her moms? I’m at a loss.. idk what to do.