Hello ladies

So, our wedding is in September and we're having a cocktail reception. It's on a Sunday and it's from 7-11:30. That being said, we decided pretty early on that we didn't want kiddos at the wedding. FI is away right now so he told me aok on the kiddo stuff and agreed that our reception is too late for some of his second cousins (1 yr old-5yr olds) but the 12 yr olds and up would prob be ok to come. FMIL agreed as well and it was her family in question that was being discussed (my family and friends knew to leave the babies home). Well, today I find out nothing was ever said to the families with youngins even though this was something FI, FMIL, and I all agreed upon. She told me she doesn't feel comfortable telling her family not to bring their family. the last thing i would ever want to do is offend anyone and certainly I know that I could just address the envelopes to the adults, but the whole point of the discussion between the 3 of us was to limit the ages of child-guests so we didn't have this problem 2 months before the wedding and so word of mouth would be sufficient. I love my FMIL; we're great friends and she's a great person, but I just feel like I'm railroaded now because even when she's had the opportunity to say things to people she hasn't and I don't know that side of the family at all. What would you ladies do?
Sorry this is so long, I'm new at this posting thing and have only commented twice...