I have noticed that parents are, on average, more grown-up than non-parent adults. I guess it has to do with having something to protect, and needing to make many sacrifices and decisions where failure is unacceptable.

I have been to rationalist meetups with children running around — they took place at the family's home. Parents with babies might be happy to attend meetups at homes or at parks. But I don't see parents taking their older, less-easily-amused children to meetups unless they're more like church: There should be more than one child; and space for them to play while the adults do adult things; and, depending on the geography of the venue, an adult to supervise them; or a more structured activity just for the children.