Received First RSVP with Guest Written In

I am really dreading this conversation I have to have. Our wedding is at 7 pm and the reception goes until 1:00 am. FH and I decided that we did not want to have children at the wedding, even though I am close to my 9 and 11 year old cousins--practically an aunt, and FH has a niece who we love, but is at the height of the terrible twos and we have friends who have kids whose birthdays, christenings, etc we have attended. We debated about if we should have our invitations state somewhere 'adult only reception' or not. We looked in etiquette books, talked to friends and the conclusion was that since the invitation emphasized that it was formal; at night and that the invitations were addressed only to the people who were invited that we shouldn't have any problems. WRONG.

Our friends have a seven month old and we just received their rsvp with the baby written in as an additional guest. What is bothering me about all of this is the fact that the couple didn't call us or email us. We see them often and in fact, we are going to a baby shower that they are hosting for another couple tomorrow night. It's so ironic too because I just had a conversation with my FSIL who has the two year old, about not inviting children and babies to the wedding. FSIL said that she personally had no problem--it meant that she and her husband could have a good time, but she warned me that there would be people with infants who would want to bring them. I suggested perhaps not because the wedding was formal, late in the evening..she said don't be surprise. How right she was...

So any recommendations for the best, delicate way to handle this situation? I now get the feeling that it will definitely come up again and I just can't make any exceptions. We have a few friends with kids whose age range between 1 to 11 who have rsvped and are not bringing their kids. If we let one couple, then we have to let all of them including FSIL. I don't know if it is a matter of not having the money to spend on a babysitter or if they have never left their baby with a sitter at night. Would it be wrong of me to offer to pay for a sitter? I know that could possibly open a whole another can of worms...

The lesson to learn so far is if you really do not want children or babies at your wedding, putting on your invitation that it is an adult only reception is probably good etiquette after all.