Not sure if theres an exact cutoff age, but I find mothers taking toddler boys into the womens room is well accepted but not the opposite. Women are assumed to be less predatory, obviously.
Hell, I didn’t go into a mens room until I was over 18! and then only in an old school which was changed from a school to a cultural center so no one was in there.
I’d say that about five or six should be the cutoff age for those without disabilities. After all, that’s when kids start school, if they haven’t been in preschool or preK, and they need to be able to use a public restroom by themselves.
I guess I’d consider the aforementioned firebugs to have mental disabilities. They should probably be kept on a leash at all times.
She may have special needs. She may be afraid to go in alone because of a previous experience.
Otherwise ten is way too old for a father to be that protective.