So. I've read a lot of threads on this, and even touched on my situation in another thread of my own. But.
FI and I decided to invite all family, regardless of age, so this will include children of family members. We did not include invitations to children of guests that were not family.
A former coworker and friend of mine has two twin girls, and she takes them EVERYWHERE. She has always been a proud person -- fine, no criticism there, in fact bravo! -- but her children are like an extension of her public persona.
So anyway, I sent an invite addressed to her and her husband, and not her children, and she asks if they are invited. I say no, and explain our rationale (children of family only). I also add that we didn't want to show favoritism by allowing children of some but not others. And she jumps in with "But you HAVE shown favoritism." I suspect from her body language that she is offended, at least a little.
So, ok. I see one of my mistakes was to even try and justify, or explain, why some children were invited but others not. I know I don't need to, and probably shouldn't have bothered. I should have just said, "No, I'm sorry, your children are not invited." But. My fear was that she, and some other guests with children, would arrive at the ceremony and see other children present, and feel angry about being discriminated against.
I see the rules of etiquette, and then I see common expectation. They can be a bitch to reconcile. Advice?