"surely he should have taken her into the mens changing room"...because obviously the girl wouldn't mind a group of men looking at her. I'm sure the Dad wouldn't neither!!
So if you take a small child into a men's changing room, they will all perv on the child, is that right?
Rather than, um, not giving a fucking shite about the fact that a child is there.
When a woman brings a little boy into the women's dressing room, is that a "group of women looking at him"?
If not, why not?
Men's changing rooms are where men go to get changed. They are no more full of "hairy men waving their willies about" (like some idiot Dad said earlier) than women's dressing rooms are full of women showing off their boobs and hairy fanjos.
Adult bodies (of either gender) are not disgusting and there is no more reason to protect a small girl from seeing a man get changed there there is to protect a small boy from a naked woman.
Single-gender changing rooms exist for many reasons, some good and some bad.
But where they exist they should be observed by ALL adults and older children (depending on the rules of the place) and any younger dependants being brought along need to go with the gender of the adult.
I can't believe people think that little boys should be in the women's dressing rooms but that it's unacceptable for little girls to go into the men's.
What are the fathers of daughters supposed to do?
Can girls only be brought swimming by women? 
If men's dressing rooms are so dangerous that children can't be brought into them supervised by their own father, then we maybe need to start locking all men up as soon as they grow pubes.