That's oversimplifying it a bit. The state does get
involved when there is a very strong claim of harm to the child. That
also the root claim of anti-abortion stances.
are the children claiming "harm" ?
are the parents of the children claiming "harm" ?
who has the legal standing to claim that someone unrelated to them is being harmed ?
Legal standing is whatever a judge says it is. They made up the concept out of whole cloth. They would claim the government has a compelling and narrow interest in preventing the abuse of children. That's how social services work.
If children always knew when they were being harmed and reported it, they wouldn't need so much protection. They don't.
This is permanent mutilation before the age of maturity. I believe strongly that a great number of these children will decide between the ages of 25 to 35 that they were indoctrinated, lied to, and abused. They will suffer serious psychological harm. At @thett3 points out in there are already some examples.
I agree with your baseline bias though. It's a very slippery slope letting government (or anyone else) interfere with interactions that are fully consensual. In the case of children though the precedence has long been established that they are not responsible for their decisions. So long as society sends that message children will be traumatized by perceived harm that could have been avoided by adult veto no matter what those children agreed to at the time.
At this risk of triggering Incel-chud this is the reason underage sex is an inexcusable risk of harm. Children are sponges, you can indoctrinate them into almost anything if you start young enough. It's not something to experiment with lightly and permanent surgery which removes the opportunity to live a normal natural life sure as hell counts as something they may consider harmful later.
It would be interesting trying to formulate moral principles for this situation.