Another lurker delurking, here. Been watching this unfold completely agog!
At this point (of beginning to wonder if she is actually temporarily unhinged), given she is a friend and you have history, I reckon she deserves another phone call.
A straight-talking phonecall where you are in charge, saying that no adult apart from her wants to have any children at this weekend. That she is making the situation very difficult, but the majority decision is no children.
Ask her if there is anything the matter that she needs talk about - you never know. If not, be firm, to the extent of saying that if she can't come alone she and her DD will effectively ruin a weekend that everyone else has paid a fortune and travelled long distances and arranged their own childcare for.
All this emailing and texting and talking behind backs (not saying in a negative way, but A is telling B what C said or did and D is now emailing A B C D E F and whoever else etc) is only going to lead to unhappiness. Offence will be taken, slights will be felt, everyone will feel bad...just be open about it!
FWIW the same just happened to me: I could not organize childcare (actually didn't want to as my baby is still too young to be left for four or five nights without me - meet-up was a 13 hour flight away), but all the other girls either have older children or no children. Totally gutted to have missde out on what seems to have been a great weekend, but it was my choice to have a kid. There will be other occasions, as there will be for your friend.