I work in an adult gym-no kids under 5 of the opposite sex. There's no reason for them to be in there because it's not a facility for kids-they stay in the kids area. I also worked at a Y that had a preschool attached. The Y divided the ladies locker room into the kid/family side and the Ladies side with a shower curtain. Personally once my son was 5 he went into the men's locker room by himself to change. If he was too pokey I sent a staff member in there to break up the party. IMO any child over the age of 5 should be able to dress themselves in their gender specific locker room.
In my opinion too. But I don´t think it´s just a question about whether they´re able to or not, but more a question of whether you trust the people around them in that locker room or not?