Really other arrangements need to be made. First off the adult needs to make sure bathroom is clear so nobody one is surprised. However this is really not totally possible in a public shower scenario. While the woman may initially clear the bathroom others may show up while 12 year old is showering. Not expecting any males to be in the bathroom others may unknowingly display themselves.
A better option would be to take the boy to the men's bath, If the mom needed to accompany him the only person she would put in harms way would be herself!
While she may see some junk no one else would be affected, there would be less collateral damage by taking a boy to the men's room vs. the ladies room.
Even then it is not an ideal situation.
In the end privacy for the public needs to be maintained and for the most part it is. I have never encountered the opposite gender while using public showers. What the OP describes is a unique scenario