Parents making major medical decisions has a huge precedent in a wide range of procedures with significant risks and consequences. Separating conjoined twins for example.
It is also entirely clear that there have been parents (usually mothers) who wanted to have a trans child because it was cool.
The passive voice language here is really bizarre. Who is doing the assigning?
>Separating conjoined twins for example.
This is not, in any way, comparable to a 12 year old taking medications that permanently sterilize them.
That suddenly looks like a very silly argument, doesn't it?
At 12 kids do not have sufficient capacity to handle any major decision, including any medical procedure.
That does not take away their right to see their best interests represented and defended.
Of course, the next best thing (if a decision can't be made now) after stopping time are puberty blockers. Which are not completely without risks, but this applies to the other two options just as well (if not more so).
You can't not make decisions, and to claim so is to frame choosing one particular option as not-a-decision.