This has me so angry on your behalf. When you divide the cost my the number of people it usually makes it cheaper! Especially if you make the kids pay their share :D

I think you're right to let those relationships go, but I'll diverge from you in thinking that they're actually just not very good friends. I have friends who are parents. Not many, but some. I ask about their life, they ask about mine. I ask after their kids, they ask after my cat. It's equal. That's real friendship.