How does one handle someone bringing a baby to a kids-free wedding?

I didn't know what category to put this in, but my partner and I opted for a totally child-free wedding however we have a guest who has simply decided her grandchild ( a 23week baby) will be coming along - without asking anyone! The parents are invited to the wedding but only one set of grandparents. I know the other set is available to babysit (and would be delighted to) but the guest is simply insisting that the baby will just come along (and note - it's not the mother insisting, it's the grandmother). How do I have a conversation with her or at least my M-I-L (it's her cousin) to let her know that it is our wish NOT to have children attend - and there are other couples that are respecting these wishes! I think it's rude both to us and those guests that have respected our wish to simply assume that this is ok.