How to tell my friend that her kid isn't invited?

You answered your own question.

Call and explain that it will be an adults-only ceremony and reception.

Just call her.

"Hey I got your RSVP back. I'm so happy you're coming. But we are having an adults only wedding."

If she says she can't make it because she won't have a sitter.

Say, "Aww man. That's disappointing. I completely understand though. So what's been going on with you? How's your mom (or whatever other family member you know)?"

This doesn't require a whole lot of explaining. In an email I feel like that's what it will turn into.

I would call, and simply say "we are so excited to have you and ((husband)) join us, but I did just want to let you know that we are having an adult reception. But hope that you and ((husband)) are still able to attend!"

I would call, and simply say "we are so excited to have you and ((husband)) join us, but I did just want to let you know that we are having an adult reception. But hope that you and ((husband)) are still able to attend!"

"Thanks for rsvp'ing! We noted your son was included. As much as we would love to have him come, there will be no other children at the wedding. He will most certainly be bored! Hope you both can still make it!"

Call/text/email....it's all awkward haha! I would just tell her that you got their RSVP, but unfortunately it's adults only, and you still hope she and her husband can make it! Done and done.

We are a bunch of great minds thinking alike today!

Ugh we had this happen. It is an awkward conversation, but one worth having. You can call or email.

@Alecia - thank you for saying that - it gives me a little piece of mind! Thank you all for your very helpful advice. I'm going to try and make it as un-awkward as possible. @S&J - I hope they can still come. They're local and the wedding isn't until October - I think that should be plenty of time to arrange a sitter!

I'd say call also, people have to respond immediately when your on the phone with them. When you email or text that gives them time to think and possibly be angry or have a much different response. At the end of the day, it's your wedding and having a 5 year old there and all that comes with a 5 year old isn't worth the trouble. I have a friend with teenage twin boys and they go every where they go and are stuck to them like glue, however I had no problem saying adults only and their almost adults and probably wouldn't cause an issue and some of my wedding party are young adults the same age.

Call and explain. That's all. BUT, in my opinion, idk that the babe in arms should be there either. Do you want a crying baby (possibly could happen) at your wedding or reception? I get the whole breastfeeding thing as I breastfed my son but there are other alternatives such as pumping before and having the sitter use a bottle for that night. Just my opinion.

I actually just had to do this yesterday. I explained to her that it's adults only (Other than Flower Girls) and that i'd already told a few people that they couldn't and I had to be consistent, but I hoped that meant she couldn't come. She was super cool about it and said it was no problem and she'd find a sitter. Hooray for small victorys!

@MissWtoMrsH - the friend bringing the baby is from out of town, and they're already spending money to get to my wedding. I don't want to also ask her to pay for a sitter, who will almost certainly be a stranger, to watch her infant. My friend is the type of person who will walk outside if the baby makes even the smallest noise during the ceremony, and receptions are noisy anyway so that doesn't matter to me!

@MissWtoMrsH, you cannot tell a nursing mother they cannot have their baby. OP is right.

You just have to call your friend (I would call, not text) and just say "hey! I'm so sorry for the confusion - we aren't having any children at the wedding. [Don't mention the nursing mother]. I hope you and husband will still be able to make it!" Keep it short and sweet, without justifying your choice, and pretend like it was your miscommunication, even though it was actually her's

@A.L. - thanks - that's what all the breastfeeding and mom-blogs told me. Also, an infant won't take up a seat, eat food, run around the reception, take advantage of the open bar, etc. Thanks for your advice @MARY, @Cassondra, and @Matt!

@annakay511 - that's great advice - and you're totally right that I shouldn't justify because they are two entirely different kid-demographics.

Even if she chooses not to come because she's butt hurt about your decision or just doesn't want to get a sitter, you gotta respect that. Like most things surrounding weddings, you can do what you want but best to believe people will do the same.

@S&J - of course! Her decision is her decision, and we will respect whatever that decision is but hope that they attend. They're adults and I will hope and assume they act accordingly.

"Your kid isn't invited. "

I would just call her. I think most people would understand, they just assume their kids can come.