Maybe I'm a bit of a bitch, but I would be very blunt in telling him the kids aren't welcome. If he was told when he was invited that it was an adult party, then he shouldn't even be asking.

I had friends this year that wanted to bring their kids, and I said no, but I invited them over on another day to visit with the kids and a warning that I had some gorey.scary decorations.

__________________
Today is the Tomorrow you worried about Yesterday... now you know why.