Children can give informed consent? We’ve agreed pretty unilaterally as a society that that is fundamentally untrue.

In every context? About everything? No, we have not. Just because a child isn't old enough to consent to sex doesn't mean they can't give consent to anything at all. Maybe in the US, the only country that has not ratified the UN Convention on the Rights of the Child, where parental rights are so extreme and out of control that parents can hire people to abduct their child in the dead of night off to a Holes-esque labor camp, if not a "conversion therapy" brainwashing camp. But in more civilized countries, there's more of a balance where children are gradually given more autonomy and rights appropriate to their age.

But even in the US, I'll give an example. When I was in middle school, we were assigned to dissect frogs. I believed that the assignment was morally objectionable, and said that I would refuse to participate - I withdrew my consent. Most of the rest of the class did consent to the activity. My teachers accommodated me and a few other students by letting us do a computer simulation of it. But both my teachers and my parents wanted me to do it, the only objection came from me, expressing my own will and my own convictions.

In the US, there seems to be this neurosis that the parent-child relationship is something bordering on ownership, and there's a corresponding fear of, "If I don't own my child, then who does? The state?" This is why there is a preconception, especially among conservatives, that if a child comes out as any form LGBT+, they must have "gotten it from someone," often, they assume, through abuse. In reality, teens are capable of making their own decisions regarding how to identify, as expressions of their own will - a teen can say "I'm not into girls" in the same way that I, when not even a teen, could say, "I'm not into dissecting frogs."

Whether, and at what age, children can consent to things like ballet classes, Adderall, puberty blockers, and other forms of gender affirming care are valid questions to examine, they are not resolved by this oversimplified way of thinking that the ability to consent to anything flips on like a light switch at age 18. In the case of gender affirming care, not only have these questions been examined, and generally answered more in favor of the child's autonomy, but they are also reexamined for each individual child on a case-by-case basis. Doctors are both careful about and (generally) supportive of gender affirming care.