I have to disagree here, I don't think there is anything wrong with asking a kid to wear a sport specific garment and dress appropriately for practice or competitions. I agree that there is no need to be overly controlling in implementing that rule, but it really isn't any different than any other sport. Soccer players are expected to show up in cleats and clothing that allows for ease of movement, swimmers need to wear proper competitive swimwear- things that aren't necessarily a safety measure but allow them to best participate in their sport. I don't see gymnasts being asked to wear a leotard or something similar as being any different.
I agree that there is no need to be ultra controlling or overly specific based on the preferred taste of the coach when it comes to the small details (no shorts, no bra straps during practice, etc.), but I really don't think asking kids to wear a leotard is too much. And yes, there should be space to allow girls to be more modest if they choose, but a line needs to be drawn somewhere.
I never really understood the gyms with very strict dress code guidelines until working at a gym with a very lax dress code. You give them an inch, they take a mile. Not so much the little ones, but the ones whose parents trust them to dress themselves for practice. Baggy t-shirts, drawstrings flying out of shorts while on bars, baggy shorts or pants wrapping around bars (or not allowing them to shift around the bar properly), flowy tops getting caught on a coach's thumb while being spotted- those things matter.
There is equipment and dress that is needed for safety (think cleats, guards, mouth piece) and stuff needed to do your "thing" (grips, toe shoes, tap shoes, chin rest, your instrument, bat, club etc..). Clothes that make sense for the sport, clearly a swimsuit for the pool.
And there is a difference between competition/performance dress and practice dress. In many things. As a kickline back in the day we didn't practice in our kickline dress, nor my twirling costume. Dance, you don't practice in your Swan Lake outfit. Marching Band, no don't wear the heavy jacket and hat with plumage to practice. My kid doesn't wear her black and white orchestra dress to every practice and lesson. We/they have dress rehearsals for getting comfortable with what you actually wear.
Even the pros don't do full suit up, having been to Giant and Jet training. Those dudes wear their safety gear, pads, cleats, mouth guards, helmets and such, but no not full dress...... at practice. Scrimmage day/practice they have required dress, but its a looser code for just practice.
I get the need for not wearing loose clothing for gymnastics. But the leo only thing. Just not necessary. My kid really is not a leo only kid. She gets there is a required uniform at meets and deals. But for practice she wants to be in her comfort zone. And that's currently gymnastics shorts with her leo. Our (coach and I) biggest worry when she first started team was getting her into a leo, because she was a tank top and shorts kid. For years it was biketards and the first day she had to wear that as a 6 year old was a fight. She is neat and safe and comfortable.
And we are lucky, both gyms we have been at aren't so rigid. And I get there are gyms that are way more rigid and controlling, fine your house, your rules. But its not about "needed or else your performance will be compromised"
So no I just don't get the leo only thing.