I used to belong to a gym that is in a hotel, so it has paying members plus hotel guests. You have to walk through a locker room to get to the workout floor. There is a sign at the front desk saying no boys over a certain age (4?) are allowed in the women's locker room and a male employee will escort them through the men's locker room if there is no adult male with them.

I once changed my clothes in front of a boy who was about 10 because his mother and grandmother decided he should stay with them. And not just to walk through the locker room, but to stay there for several minutes using a locker. I didn't realize it wasn't a girl until after I gave him a full-frontal topless show. I later told my sister about it, and she said her son who was about 10 at the time would have been freaked out if she took him into a women's locker room like that.

As for the woman who thinks it's okay for her son to see a naked 10-year-old girl in the shower, I would have filed a complaint against her with the gym management. That is NOT okay and if she doesn't understand the impact on the girl then she should lose her membership at the very least.