You aren't going to like my opinions but that's never stopped me before so...
A/A were way out of line in their comments to Mishka about his wife and child. They've probably damaged their friendship with him beyond repair. You just can't expect a friend to be okay with your open hostility towards his wife and child. He might be able to tolerate their dislike of his wife so long as they were polite to her. But to reject his son is to reject him in a way.
Without ever having met Mishka, I'm willing to bet that being a father is a big part of his identity. Part of being a good father is sacrificing your own desires in order to give your children what they need. That Mishka, despite his misgivings, decided to man up and do it again at this stage in his life is to be applauded. A lesser man, a more selfish man, might have walked. So how did A/A respond to Mishka's sterling display of character? They whined that he couldn't come out and play. Hence his comment about their lack of support.
I don't think A/A should have necessarily changed their vacation plans to include Mishkha's child. But telling his father they resented the boy's presence in his life was a big mistake. They'll need to get over that resentment if they want to retain Mishka's friendship. And for the record: I'm also in my 50s and childless by choice.