I see where you're coming from... unfortunately, not everyone has a large, air-conditioned facility to practice in. Rather than having people pass out from the summer heat, I don't think it's killing anyone to see them practice in sports bras. Anyone who ever went to NCA College Camp at UMASS back in the day would remember the extreme heat in that gym. I was extremely self-conscious about my body, but still ended up in a sports bra at that camp lol.
Is it really THAT much more skin showing than a uniform top? A bathing suit? If you bring your kids to the beach, they're gonna see girls in bikinis walking around where there are men of all ages as well. I would find it difficult to believe that most girls who cheer aren't out on the beach in bikinis. I am fairly certain that I was usually the only girl sporting a one-piece at our high school/college functions at a beach/pool. That's a leisure activity, so there's no immediate necessity to wear a certain thing... but training in a gym is an athletic activity, requiring functional athletic wear.
Sports bras are worn by female athletes in every sport, it's not like they're naked. If the kids are comfortable like that, I don't see an issue. jmo