Ok, so here's some background:
I have 26 first cousins - we are all pretty close - and I am the youngest, so several of them have Middle school / HS age children. I know a lot of the kids and I'd really like to invite them, but it would double our guest list which we just can't afford. So, our official line is "no kids of any kind."
My mom suspects that two of my cousins who are sisters want to come with their College-age daughters (wedding is far for everyone). I'm not close to these particular girls - some of my other cousins have kids to whom I'm closer. One of these cousins is married so i'll invite her and her husband. The second is divorced so I will invite her with a guest. So, what if her guest is her daughter? What if the first wants to ditch her hubby and bring her daughter? Note that the daughters are good friends.
On the one hand, I don't care as long as it doesn't increase my numbers overall, but should I allow the substitution game??