I'm not sure that you really are understanding the issue here. Not sure either why you decided to bring up off-topic issues and questions unrelated to the OP? Of course the thread is focussed on helping OP navigate this difficult issue.
I am surprised that you don't see the slippery slope in a coach wanting to reduce the butt size of an 8 year old who is not overweight? Of course this is body shaming as a) weight loss is not needed and b) how could this even be achieved? It is likely genetic and spot reduction is not possible anyway and would cause a loss of muscle and power overall. It would also be body shaming to tell a low-BMI NCAA gymnast that she needs to make her butt/breasts/thighs "prettier" by dieting.
To be crude - if you look at the "Final 5" all of them have different butt shapes and sizes yet all of them were very successful gymnasts. So coaches who have very rigid body-shape requirements would have turned down an Olympic medalist! Note! I am not talking about overweight gymnasts at risk of injury at all but girls who have different genetics and muscle/fat distribution. The US team in recent decades has shown that you don't need to follow the Russian Ballerina/Romanian Waif model to succeed.
If you truly do not understand the effects that this sort of body shaming about their genetics can have on very young gymnasts please read/re-read Katelyn Ohashi's blog and diary excerpts regarding body shaming and disordered eating. That she was verbally abused for not having the preferred body shape for her gym and that the body shaming was relayed to her mother who instead of taking a stand against it colluded with the coach and reduced her food - which in turn led to a miserable existence and disordered eating patterns.
https://behindthemadnesssite.wordpress.com/2017/08/24/dear-standards/