I would not underestimate muscular tightness - I had chronically tight thoracic muscles during puberty and it did some damage to my ribs / mid-back.
I'm sure the gymnasts could incur damage that hurt growth. Though AI says the gymnasts reach predicted adult height. The child weightlifting/stunted growth hypothesis of course has been disproven, though it never made logical sense to me even as a child. I always thought of how much load all healthy kids placed on bones just by jumping around. Now extreme overtraining coupled with malnutrition and delayed puberty I guess could do a number on bone calcification or whatever, but I don't know if that has been looked at. Nowadays they probably make sure the girls get enough calcium and other micronutrients.
I know the U.S. historically imported eastern European slave driver coaches but luckily today child protection is better in the west at least. But it used to be that countries like Sweden didn't have a chance in hell of medaling in gymnastics as it required some child abuse.
I had a lifting phase at 9 years old and if I'd advice another kid or myself at that age I would avoid overhead lifts, wouldn't worry as much about lower back. I remember a lot of shoulder joint pain
