Maybe it's just that I personally was raised with zero attendance at anything remotely that structured, but it's hard for me to imagine kids sitting through all those highly abstract stories, many of which rely on lots of background concepts, and being anything but bored stiff (and probably annoyed). Am I wrong?

(shrugs) You're not wrong, but I'm not sure you're right either.

In my own case, growing up as an Orthodox Jew involved sitting through lots of highly abstract ritual observances that relied on lots of background concepts (and frequently being bored stiff and annoyed). And if a rationalist group is only as successful at retaining the involvement of parents and their kids as Orthodox Judaism is, dayenu. (Which is to say: that would be sufficient.)

More generally, I suspect that it's perfectly possible to involve kids in something that structured, it just requires giving the kids roles they can engage with in that structure.