I hate to add such a spin to this post, but coming from a background as I did and having numerous children, I can't help but chime in.
Out of college, I worked with teenage rapists. They were both victims and victimizers. They were under the age of 18 and were remanded as young as 12. Many of them molested siblings that were as little as one year old. The abuse and behavior stemmed from abuse that usually started at home, within their families, the last place anyone would expect it. All that said, these individuals sought out places where they could find their victims and even many staff I worked with sexually assaulted the kids in the home. The most savvy predators were extremely smart, IQs well over 130 and were calculating and socially adept enough to trick their victims into compromizing situations and often felt uncatchable. As a staff you are able to read the bios of their victims and the various stories, etc.
That experience made me leery of things the average person would never think of. I even study relatives with my children for that reason. Considering we are doing the best we can to protect our children, we shouldn't be criticized if we err on the side of caution and bring an 8 or 9 year old into a stall with us. Even minor groping is a form of assault and that happened to me on a regular basis on the NYC subway, you can't get much more public than that. A child can be fondled or groped within seconds and it could be more damaging than one might think.
Also, I attended a college where our dorms and bathrooms were unisex and girls could be showering in a stall right next to one of the football players who lived in the room next door. It was not a big deal at all. Considering there are stalls in the restrooms, I think it is much more acceptable for a boy to go in the womens bathroom than vice versa and I don't see what the issue is.
Mom's do what you feel is best because once something happens, the mark that is left on your child could be indelible.