I have two sons. I remember when my oldest was 10 and I was debating the wait outside the men's room door, or take him with me option.
Right around the time that I was in this place in my life with my son, a news story broke about a ten year old boy who entered a men's room alone at a beach resort and was badly assaulted by a man already inside. Horrifyingly enough, he left the boy in very bad shape, so that when the man exited, he had ample time to get away before the boy's family went in looking for the boy.
I can't imagine anything worse as a parent than to have your child harmed so horifically. My heart still breaks for that family. I know that these tragedies are random, yet every time my younger son has to use the bathroom, I still take him with me. The "what if" is just too possible. You really don't know who is inside that bathroom.