uninvited guests ** WITH UPDATE

UH OH - I got my first response where someone wrote that their baby will be attending (when it specifically said 'Mr. and Mrs.' on the envelope)
I have to call her and address it - just wondering how other people have handled this.
It's not an option - I have 30 cousins and 20 second cousins...waaay too many to not set an age limit (which we set at 13). We can't make exceptions, so any help with dealing with this call would be greatly appreciated

UPDATE** I spoke with my cousin and she was totally understanding. She said that she was already trying to make arrangements, but said that she would definately get in touch with me if she needed help. I am soooooooo relieved!! Thank you all for your encouragement and advice