Children are not adults. Even clever children. We don't allow them to make decisions that can affect them long term because they need protection.
A child looks to their parents, peers and wider society for cues on how to act, what is ok and what isn't, etc.
A young Indonesian girl who is raised to believe that if she isn't circumcised she is unclean is going to believe that is necessary. And she will go to a hospital and have a procedure done. It's lower risk than what happens with rusty knives and acacia thorns in other countries. Is it somehow ok now because a: the child is clear and emphatic that she wants it done and b: it's in a clinical setting?
No and no.
It is harmful, and reducing the risk by putting it in a clinical setting still doesn't make it morally right to carry out the procedure. As mentioned before, a child isn't legally allowed to have a tattoo, and they're carried out in sterile environments too.
If a young girl has had the weight of social expectations on her, the weight of "gender" on her, is actively encouraged to transition by minor internet celebrities and an online army of enablers, is she, as a child, able to make a free decision to have a double mastectomy at 13 (in the US, this is happening)?
Again I say no. She is a child, she can't consent, and she really should be educated about all the competing interests trying to influence her thinking and her self-image. Then as an adult, she can make her own decisions about most modification, transitioning, gender presentation, name changes, etc.
What's hateful about that?