Moms of having a boy and girl child

Is it normal for your boy child and girl child to “explore” each other? My friends has caught her 2, girl age 4 and boy age 6 kissing, and no clothes on and boy child has had a hard penis when being caught. They say that girl child has sat down on his penis and girl child had touched penis as well. She said that this is the 3rd time that she has caught it and she said that she has talked to them about how their body is for them only and no one sees them or touched them. That they can kiss on the cheek but not kissing the way they were. She said that they see her and her husband kiss but they don’t make out, she has no clue where else they would have been exposed to such stuff and she doesn’t know what to do. She said she feels disgusted and like a bad mother from it.

I have tried to give her as much advice as I could but I only have a 10 month baby boy so I have no clue what else to tell her. So I told her I’d come on here and see if any other moms have went though this and what they have done and what’s “normal” Don’t bash her, she’s already bashed herself enough about it.