I am unsure why it’s important that children are involved. I agree it’s the parents choice. If it were my kids I would want to know what the point was and what the book was. Beyond that meh.
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.