It's not for the benefit of the kids. It's really for the benefit of the parents and the drag queens. One of the pictures posted above, you have drag queens showing (and I assume they read it) a book about a kid who's a "drag princess" (I don't know what other term to use). I am sure the parents are really proud of themselves.
Personally, instead of spending an hour to take my kids anywhere to have anyone read to them, I'd rather spend the hour and read to them myself. As for "diversity exposure", my kids are exposed to diversity every day.