I’m worried too. This is not normal 6-year-old behavior. It’s also not normal for a 6-year-old to use language like “humping.” If he saw it on “TV,” he’s been watching porn. Such things are not on network TV during the hours that kids are awake. Then again, if that’s a cover story, he may be getting the information from the nephew.
As difficult as such a conversation is likely to be, you and your husband need to have a calm and clear discussion with the kids’ biological father and his girlfriend as soon as possible. Grandmother should also be included. Avoid blaming each other. Keep the focus on figuring out how the kids may have been introduced to adult sexuality and what you are going to do about it now.
Protecting our children from inappropriate sexual contact is every parent’s responsibility. There are four very young children in your collective care. All of them deserve to have a safe and healthy childhood. If any of the kids have been exposed to something they shouldn’t, they may need the additional support of a counselor.
I hope the adults can put aside any differences that may exist among them for the sake of the kids.
I wish you all well.