I was pondering on whether Gillick competence is ever likely to apply when dealing with treatment for mental health disorders, but there is a double problem here.
A child can presumably give consent to treatment for anorexia, but the treatment for anorexia involves doing something the child doesn’t want. Their mental health disorder is driving them the other way.
With transitioning, the child generally desperately wants the treatment. You can explain it to them all you like, but I don’t see how you can ever really assess whether they fully understood the consequences, but just agree because it’s what they want.
If a girl wants contraceptive pills, it can be ascertained that she understands. She also wants them, but presumably the doctor assesses whether she is also rational at the same time.
I don’t think we can assume any child with gender dysphoria can be truly rational about weighing future side effects against what their dysphoria (or peer or parental pressure) is urgently telling them, right now. I almost feel that having a diagnosis of gender dysphoria more or less disqualifies you from being considered rational enough to assess consent.
Which is all related to the fact that affirmation is basically enabling (or even encouraging) a mental health disorder, rather than trying to control or moderate it or its effects.