Have you checked out the Family locker room? If is it same( with all ammenities) as the single sex ones, I can see your frustration. But many times, these rooms are just that, a bare room, tiny, only space for one pair of people at a time, many times they have no toilet or sink, they were added as an afterthought. Maybe it was occupied, there was a long line, maybe her son needed to go potty, or needs help changing, many boys do still need help at 6, especially with wet swimsuits. Put yourself in her shoes, would you send your daughter into the women's locker room alone, without you, and expect her to be able to navigate the whole process of changing and pottying and everything and come back out with everything she took in there with her???
Our Local YMCA says no boys over 5 in the women's locker room. Thankfully, they do have a special needs/family room for changing with older boys ,who for whatever reason you don't want in the Men's locker room alone, off to the side in the entryway to the Women's main locker room, but around the corner and out of sight of the main female changing area. But there is no toilet in there, no sink, just a smallish room with 6 lockers, a bench, and a door. Unlike others have said, there are no "stalls" or curtained off areas to change in our locker room - I don't see a probelm at all if that is the style of room it was, you still have privacy then, for the time you are naked. There are 2 small stalls that have a toilet in them - but you don't wanna hog those while others may need to pee, and 4 curtained off shower stalls, but they are always wet and slippery, one shower area has 4 showers right in a line with no curtains, so everyone usually is buck-naked in the middle of a large room lined with lockers on the walls. We are also lucky enough to have an 18 and over women only locker room within the regular women's locker room - it has the sauna in it and has a combo lock on the door. Unfortunately lots of places don't have the luxury of so much space to designate for different people to change in.
I don't know, 6 doesn't seem that old to me, it is a scary world out there, and without a trusted adult man to go in the locker room with my son, I wouldn't necessarily want him in there alone till 8 or 9. I understand the privacy issue, too, and people do need to teach their kids locker room manners - it is rude to stare, point and comment on other's bodies - pay attention to yourself, get changed and get out. But it is a slippery slope, where DO you draw the line? What if you had a 6 year old boy, would you send him into the men's locker room alone, if he had to go to the bathroom, or to change if the family locker room was full???