I think this thread has become quite partisan in its references to "precious boys" and "little princesses".
All children, girls and boys, are precious and all children deserve to be safeguarded from abuse. They also deserve private spaces where they can change in privacy and with dignity, without being made to feel uncomfortable.
Women and men deserve this too.
The problem is that logistically it can be difficult to achieve this for everyone, so the question then becomes, who do you prioritise?
I think the most important thing is that no adults should be in opposite sex changing rooms. It is unacceptable for adult men to enter the women's changing room with their daughters; it is equally unacceptable for adult women to enter the men's changing-room with their sons. That is more unacceptable than having opposite sex children of any age in the men's or women's changing-room. Adults have a greater expectation of privacy, particularly from adults from the opposite sex, than children do. So it is 100% unacceptable for adult men or women to be in the wrong changing-room.
Then I think we'd all agree that little children can be in opposite sex changing rooms. I don't think anyone is particularly bothered by a toddler or a reception-age child of the opposite sex and children of up to 6/7 might conceivably need parental help getting dressed.
There is then a grey area for girls/boys accompanied by an opposite sex parent between 8 to around 14 when they are probably mature enough to change without help but not to deal with the potential risks. And here it is a question of priorities... women and girls are prioritised in their changing-area, men and boys in theirs. So if a parent (for what is usually a very good reason) isn't comfortable with their child going in alone, it's up to them to find a work-around.
It is a gendered issue, but not so much because those "awful boys" are invading women's and girls' spaces (it's usually the mother making the decision to do this) but because, like so many other issues in society, it arises largely due to the prevalence of male aggression, abuse and entitlement towards women and children (of both sexes). Yes, boys grow up into men, but an 8 year old boy has much more in common with an 8 year old girl (they are both children) then with an adult male. But still both are starting to need privacy from each other.