I've got 2 young kids, is this really an issue. I mean if there are a series of size 4-5 plain red t-shirts does it really matter if they are marked boys or girls. If there is a top with dinosaurs on it, or George from peppa pig and then a girl wants to wear it then so what.
I regularly have to take my daughter into the mens toilets, as i cannot go into the ladies, i dont think they are teaming with pedophiles, who are watching the youngsters entering the cubicles.
We already have mixed gender toilets in a lot of places, they are usually single cubicle lockable toilets.