OP. I'm sorry that you feel like you do, but things might not be quite as bad as you are imagining. Yes some of the mothers may be friendly, and have a social life together but it doesn't necessarily mean that their children are best friends.
My closest friends now are the mums I met when my PFB went to nursery. Our children had play dates because we liked each other's company. As we slowly returned to work we helped each other out by picking up each other's children when another of us was late/busy. So the children came for tea more due to necessity. We still do have occasional outings, however, as they have grown up our children have chosen different friends.
The girls you are talking about might feel they are just as good friends with your daughter as with the others. My DD is now closest to girls whose mums I don't know at all.
Also as mums returned to work, going round for tea fizzled out. I suppose work leaves less energy for entertaining children! Great if they're invited elsewhere though! Now In year 6 it's quite rare but the girls occasionally go into the village in groups for an ice cream together. No parental involvement at all.
DS is at secondary school and has his own, completely different set of friends.
I suppose what I'm trying to say is don't despair. Even if the after school stuff isn't happening at the moment it will come as in-school friendships develop.