I'm cringing writing this, it's embarrassing and outing, so I've NCd... but DSis already knows I'm here waves and will still recognise me!
At 9, I was the only child in my class to have underarm hair. DM wouldn't let me remove it. It was impossible to hide from the other girls while changing for gym, so the whole class (and the neighbouring one, and the year group below us...) all knew. And it was of course on full display during mixed sex swimming lessons.
The other children would find reasons and ways to touch my armpits - while using a float, climbing out of the pool etc. I hate that it happened, but they were children; the adults in the situation, who saw what was happening (or were told by me) and did nothing to stop it, were fucking cruel.
25 years on, and I still struggle not to weep from shame remembering those lessons. I hate my underarms, so much so that it has made working in a short-sleeved uniform almost unbearable. I loathe my body hair, and panic if I think strangers might see it.
It is foolhardy to think that because they are "just children" they will be unaware of the differences between their bodies, and unaffected by any resulting comments // actions. I wasn't protected from my discomfort by single sex changing facilities, but I know it would have been even worse to have to change in front of the boys in my class, I can still quote verbatim some of the things said to me by them.
OP, I feel for you and your daughter, and for all children who are watching their privacy eroded by adults who ought to know better.