How often do you actually see someone's genitals while in a public restroom? I can't think of one instance in my life where I saw someone's dick or even ass in the men's room.
There is no easy solution here. While I get the complaints and wanting to force people to use the bathroom based on their biological sex, we also have to keep in mind that a trans person who has glued a dick over their vagina or something usually presents themselves as a man, with facial hair etc. Seeing what a 10yo girl perceives to be a man, walk in to the women's restroom, is going to be just as shocking as what you complain about.
Seriously, if you weren't able to see their genitals, you probably would have never known they were trans. So what 10yo is walking in to a public restroom and seeing dicks and vaginas being displayed? I would say none. Therefore if the people are legitimate trans, it would be more upsetting to the child to see what looks like a woman using the men's rest room, or vice versa. Unless you are pointing and staring at their junk, why would a kid be upset that a male looking female, dressed and conducting themselves as a female, came in and disappeared in to a stall?
It's almost as if you didn't quite think your post through enough and are advocating for something that would cause even more problems or questions from that hypothetical 10yo. That kid is immediately going to be like "daddy? Why is there a woman in the men's restroom?" (upon seeing what looks like a female in the men's restroom). Whereas if the transman is in the female restroom, or a transwoman in the male restroom, nobody will know the difference unless they are peter gazing. Especially a 10yo.
There's no easy answer here.