Something else I wanted to add, re: the focus on "what's the issue, these are really young kids so there are no physical issues", that I also posted on the AIBU threads (Is it bad form to C & P your own posts a bit? dunno, but I wanted to add it bc I think it's so relevant to all this...)
Yep. The one I've not shared - and these were boys, not transgirls, but frankly what matters to me is that they were male (what's in someone's own head or what they're wearing makes no meaningful difference to me when it's their body that can hurt me)...
When I was somewhere between 7 and 10? (AKA Brownie age?), I had a number of close encounters with aggressive boys in my year group at my (tiny, really well-run) school.
One was always trying to intimidate the girls - he was absolutely, unambiguously quite a bit bigger and stronger than us, really male-ly stocky, with this brute animosity, and he clearly relished our wariness. I remember how unsafe we felt in his presence, and how we adapted our behaviour around him. (I took him on once in angry fear - pushed him a little - and was a heroine for weeks!)
And another boy once took my head between his hands, and intentionally smashed it as hard as he could back into a concrete wall.
Interestingly, I do remember a primary school girl trying to physically intimidate me in the changing room, too, but it was markedly all posturing, and I experienced it very differently. She was seriously tall for our age, yet there simply wasn't the same degree of physical dominance, or anticipation of latent aggression, or sense of my own vulnerability in the experience.
I do remember Brownies and Guides as feeling very, very different because of the absence of the boys from school. Not automatically better (and one of my best friends from a very young age was a boy, and there were a number I liked or admired from a distance!)... but sufficiently different for it to quite clearly be offering something special to girls. Something that people are now arguing they should lose.
After posting this, I actually googled differences in size and strength in the sexes at 9 or so, and pasted the AI summary (which includes the excellent "some quora users" as one referenced source, but I think there's agreement, isn't there, that these differences begin pre-puberty?...):
I was thinking about this again last evening. I could imagine some of the nay-sayers coming back at it with, "Yeah, but this isn't always the case, and what does 10% matter anyway?" (sports-style counter-arguments). And then I thought of the analogy that (just as in sports, they'd never say this in the context of doping)... if you imagine someone proudly telling you that they'd dropped 10% of their body weight, or increased their muscle mass by 10% in the gym, people would immediately recognise that as a huge achievement, a really striking, tangible difference. And yet in this context, it doesn't matter?
Again, the sheer arbitrariness of what is and isn't valued as significant gives the lie to any argument that sexism is now obsolete.