Friend RSVP'd for her toddler to our child-free wedding...

Just received my bff's RSVP card today and it included her name, her husband's, and their (will be at the time of the wedding) 1.5 year old child. To preemptively answer some common questions:

1) We addressed their invitation to "Mr. and Mrs. X," and 2) Our website AND the info card included in our invitation suite specifically say "All events are adults only."

In case it matters, I've asked this friend to speak as my fake "MOH." (We aren't doing official bridal parties, but if we had, she would've been my MOH). Part of me feels making a stink about this starts putting me in Bridezilla territory, but I genuinely do not want small children at my wedding. I love my friend, and completely understand how her priorities have changed now she's a mom. To clarify - I'm not at ALL opposed to the baby coming with her for the weekend (I'm not a monster!). I'd even be happy inviting her mom to come as a built-in babysitter so she and her hubs can have just a few hours free. Bottom line, though - I'm not willing to bend on this, and I think that will be a shock to her.

What's the gentlest way to start this conversation? Or is there no way for me to approach this without ruining our friendship?