Whatever the reason, I don’t understand why the people cancelling don’t suggest a couple of other more suitable dates for the meet-up. If they valued the friendship they would do that. As for the suggestion that the OP should join in a conversation about other people’s grandchildren when she doesn’t have any of her own (which is a source of sadness to her) I say no, the friends should be the ones making an effort to not dominate the conversation with talk about grandchildren, but to talk more generally about subjects that everyone can talk about. That would be polite. It is not polite and is unfeeling for people to dominate conversations talking about their grandchildren, especially to a person who has none and would dearly love to have grandchildren.