I've just realised this is why it's important for the trans lobby to deny that sex is real. Or conversely, it's the logical conclusion of denying that sex is real.
So if we say "It's wrong to encourage children to trans out of their birth sex, they don't know anything, let them grow up first", their thoughts are "But there's no such thing as birth sex! It's a construct! Whatever the child feels, IS what the are".
If we say "It's important to teach children to accept reality" there response is "But sex has no reality".
(Rubbish of course - if there's no such thing as birth sex, why the need for puberty blockers, hormones and surgery? But there you go...)