Invited with a guest RSVP with a son?

Hi ladies - I wanted to see if anyone has had to deal with this one. We're inviting someone from my FHs family who is divorced and has an 11 year old son. She came to his cousin's wedding with her son as her guest. It was a second marriage and the brides girls and her nieces were there, so it wasn't like there were no kids...BUT he really wasn't invited. His mom was invited with a guest. Now the concern is that she's going to do the same thing for our wedding. Honestly the kid is sweet and I wouldn't have a problem with it if:

1 - FHs family wouldn't start WWIII over it (which I know they will because we already found out his cousin (the one that got remarried) is mad because her girls weren't invited)
AND
2 - My own sister's kids who are IN THE BRIDAL PARTY aren't staying for the reception. We're doing pictures, ceremony, cocktail hour, introductions into the reception and then they're going home with friends.

I think I know what I'm going to do and I'm hoping it doesn't happen, but I was just wondering if anyone else has had to deal with one like this?!

That hasn't happened to me so far. (keep fingers crossed). I invited a few couples with children, but luckily with the rsvps i got back so far, they're all very understanding of their kids not being invited. I'm hoping for no drama, however, because i AM inviting one child to the event. She's 9 and I've known her since she was a week old coming home from the hospital. She's really excited about being the only child there. She won't be lonely as long as she's sitting with my sister - her babysitter (my sister was the one begging me to invite her, so it's no annoyance to her). But I'm still hoping that there's no drama if people with kids see this one 9-yr-old at the reception. You know what though? I really don't care. They can deal with it.

As for your situation, she might not bring her son if she knows what's going on. Like that there will be absolutely no kids there and even the ones in your bridal party won't be staying. Maybe if she RSVPs and says that she's bringing her son, you can just tell her what's going on and say that you wouldn't want him to feel uncomfortable being the only child, especially since it's an Adult Only Party. She might be understanding of that. But if she DOES think kids are coming, then I could see her definitely planning to bring along her son.