Given the right upbringing, kids will do dumb things, because their brains are not fully formed. That's just reality.
I envy you your certainty about the nature of reality in the absence of objective evidence (in the context of our discussion about whether it is the norm that well brought up kids tend to behave responsibly when it comes to sex). Life can, arguably, be simpler with that attitude.
If you're lucky, it won't be your kids, but all you can do through good parenting is increase the odds.
While that is true, that same truth holds for a number of things that can also have significant impact, like doing hard drugs, falling in with wrong company, getting radicalized, falling under the influence of some cult, usw. I (continue to) wonder if you use the same approach (as you do when it comes to unprotected boy-girl sex) for those other issues as well -- e.g. by ensuring kids "in your watch" don't get alone-time that they may misuse to do drugs, don't get unchaperoned online time that they might misuse to get radicalized, etc.
Meanwhile, dragging it back on topic, if you throw young men and young women together into hotel rooms, you push the odds in the wrong direction.
So what happens if you throw a transgirl and girls into the same hotel room? Well, I have no idea, really, but I do know that things are different, and I do know that the possible outcomes that I truly care about are the exact same as if I threw a cis-boy into the same hotel room, because, biologically, they are exactly the same.
Beyond the risk of pregnancy, which is a real, though unlikely, possibility, there's just some absurdity in suggesting that somehow the regular girls in that room ought to all just act like this is all perfectly normal and pay no attention to the girl with the penis.
As far as the last, I'm with you. Having no direct RL interactions (that I know) with trans folks -- that is, they're a rarity -- I wouldn't be at all surprised if young girls gawped at a girl with a penis, or young boys at a boy without one.
But, I don't know, change? Just because I didn't see something growing up, doesn't necessarily mean today's kids also shouldn't, you know?
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In any case, apparently trans folks are just 0.3%, or so a quick google check tells me. That's, like, a tiny percentage, compared to, say, gay* numbers. I suppose that makes the issue both easier to handle and, in some ways, more difficult.
*"Gay" is a lovely word, beautifully co-opted to refer to homosexuality and homosexuals. Wonder who thought that up? The trans folks should think up some similarly cool name for themselves! You know, work up a cool vibe. "Trans" is such a nuts-and-bolts, purely functional, rough-and-ready term.