No, sorry. I knew this particular kid since pre-school when they were in my son's class. They started with all the presentation of a cis-boy and gradually the presentation changed. I remember someone's birthday party where they were rocking a My Little Pony footy-pajama-hoody with a unicorn horn sort of thing. There was one Halloween where this kid pulled off an amazing Zelda costume, for example. With all of it, it was clear that they were not a strict adherent to gender conformity. Fortunately, the school kids and parents, at least, were supportive and didn't give them any crap for it. Their dad implied that other aspects of their lives hadn't been so open-minded.
Their actual transition was in either 2nd or 3rd grade, I think, and consisted of a new first name, new pronouns, switching to the girl uniform, and using the girls' restroom. Again, everyone at school accepted it because it really wasn't that big of deal.
I get what you're saying... but ti Lithrael's point, that kid still isn't going to be subjected to the social pressures that females get. To some degree they might hear some of them, but a whole lot of them aren't going to land at all.
For example... Do you think that young male, who presents as a "girl", is genuinely going to be subjected to the same social constraints about sex and sexuality that females face? Do you think they're going to be subjected to a constant stream of conflicting pressures, on the one had a need to be chaste and not be a "slut" because "loose girls are dirty" and they "might get themselves pregnant"... while also being deluged with messaging to constantly be more sexy and to appear sexually available to males because their worth is based on whether or not males want them, and they don't want to end up as a "spinster" or an "old maid"? Do you think the pressures of birth control are going to have the same impact on that male child? Are they going to be faced with risking pregnancy or taking drugs that have unpleasant side effects, all because society views females as being completely responsible for protecting themselves from pregnancy, to the point where most females are expected to carry condoms in case the male they're interested in is too ******* dumb to take responsibility for their own sperm? When that child hits puberty, do you think their parent's are going to *shrink* their freedoms, and start limiting their ability to spend time with males without direct adult supervision, be expected to be effectively chaperoned in public and never alone because of the risk of sexual assault? Or do you think that child's parents are likely to allow them greater freedoms, room to roam and explore, and greater autonomy which is what most males experience at puberty? Is that child going to be pressured to be discreet about their bodily functions, because "girls don't fart or burp or spit"? Will they face embarrassment and worry about whether or not their pad might leak and then everyone would know they were bleeding from their vagina?
Sure, there are going to be some experiences that a young male who presents as a "girl" might face, especially if they pass relatively well. But you severely underestimate the amount of pressures and expectations that are sex-based. Like, based on actual real sex, not on whether or not we like sparkly things. Even the most tomboyish, masculine, wild young female ends up being subjected to the sex-based pressures, no matter how much people like you think they don't "look" appropriately "ladylike".
But if you don't want to go on formally-twitter, there are situations like
this where people tried to stop cis-women from using the women's restroom because others didn't believe they were biologically female. The first time I'd heard of that kind of thing happening was when it happened to a friend of mine.
The inability to discern sex from outer appearance is common enough that Aerosmith wrote a song about it 40 years ago.
First off, "Dude looks like a Lady" isn't about the inability of humans to discern sex. It's about a transvestite.
And that leads into my second point - we're EXTREMELY good at discerning the sex of adults. There've been numerous studies that demonstrate that even when you remove all makeup and hair, we can correctly identify the sex of a person from their face alone with about 99% accuracy.
On the other hand... when people make a concerted and intentional effort to obfuscate their sex and to mimic the markers of the opposite sex, yes, it gets more difficult. Sexual mimicry can and does complicate things. But that doesn't mean that we're not incredibly good at determining sex, it just means that we can be tricked by people who make efforts to present false indicators.
For consideration, we can tell an orange from a tennis ball pretty easily. But if you take a tennis ball and you layer on a textured coating, paint it orange, and glue a stem to the top, we're going to be fooled. That doesn't in any way suggest that humans are really bad at telling the difference between an orange and a tennis ball!
My point is that, from a purely biological fairness point of view, the contents of one's pants seems the least relevant of any of the physical traits for any competition that wouldn't be against forum rules to discuss.
This is where you start to sound like you're being intentionally disingenuous. You know damned good and well that the divisions in sports aren't based on genitals. They're based on the fact that humans are sexually dimorphic, and that dimorphism results in males being bigger, stronger, having more muscle, more fast-twitch fibers, greater lung capacity, larger hearts, and a femur-to-hip angle that is advantageous in running and most swimming. A male can totally cut off their penis and testicles, have an artificially constructed facsimile of a vagina... and they will STILL have all of the physiological advantages of being male.
This aspect of the discussion is not, and has never been, about genitals. Don't expect me to believe that you're completely ignorant of that fact.
Wouldn't segregation by height or, perhaps, arm span make more sense, if the goal was to reduce biological advantage and to focus on practiced and developed skill?
Sure, sure. But while you're at it, you might want to incorporate additional divisions around lung capacity, heart size, and the angle of the femur.
Or, you know, you could apply some very basic and well understood sense to the issue, and realize that all of those extremely important criteria are strongly correlated with sex, and then you can short-cut the entire process and just use sex as the dividing factor since that's what the resultant sorting is going to produce as an actual outcome.
Unless... unless the reality is that you give zero ***** about females, and you are quite happy to see females excluded from athletics and competition altogether.
It's actually not as hard as I thought it would be. It's been... at least two years now that I've been using exclusively neutral pronouns here. So far as I can tell, nobody has yet been confused with regard to who I'm referencing. Sometimes it takes an extra second of read-through to make sure I'm disambiguating, but it's been easier than I expected.
The only difference is that for normal, non-disordered people, they don't desire to be something else. Their psychological view of themselves is fairly well aligned to their actual behavior. 
Sometimes they aren't perceived as being as intelligent as they think they are, but compared to an average person, they're still accurate.