Some SFF certainly is childish.... as well as repetative, unimanginative, cliched, bland, even offensive, pulp. The thing is some horror, crime, romance, "true life", historical etc is just as guilty.
You can see where I'm going here, some of it's also original, thought-provoking, joyful, engrosing, political, exploratory, humorous, hilarious and beautiful.
Ditto for movies, poetry, paintings, non-fiction (I've read some shocking research that doesn't even begin to be worthy of
childishness). It's the artist that define their work not the tools and i think we can agree that SFF has had some amazing ones over the years.
And the best of all..... none of us would completely agree on what belongs in what category
.
Personally I think the fans all decked out in their team colours, singing and cheering are enjoying escapism every bit as much as the kid next to them on the bus reading Harry Potter.
Sorry, I couldn't help but add some semi-coherent ramblings of my own.