I can’t understand why it’s unacceptable to state some children will grow out of this.
The existence of detransitioners is an existential threat to the gender identity ideology. It means that if someone says they're trans, they might be trans, or they might not be. That throws a spanner into self-ID since it then needs to be acknowledged that some self identifiers are not trans. Which means that people will ask "well, if they're not trans, what are they?" We know the answer to that, but the public don't. Not yet, anyway.
If it's accepted that a non-negligable number of children will think they're trans when they're not, then all the early medical interventions have to stop immediately. That's because you can't be sure whether a child really is trans. No "trans kids" loses the trans activists a hugely useful human shield.