Keep in mind this is a hypothetical scenario.
I was recently very surprised to find out that many males in the United States are circumcised shortly after birth. No one had ever told me (it's not a subject I've spoken at length with anyone about), and certainly no one asked my permission before the procedure was performed on me. And yet it happens all the time, probably still to this day.
So this got me thinking, what are the legal limits to which a parent can physically modify a child? Could they turn a girl into a boy, or vice versa (I assume hormone therapy would work best when started at an early age)? Could they give them elf ears or vampire fangs? Could they replace all their limbs with state-of-the-art prostheses in an effort to create a Baby Robocop? As far as I know, all of these things can be legally done to oneself, but children aren't old enough to consent, and there are definitely cases where things are done to them against their consent by parents, with the law supporting the parents.
Let's assume that any modification has no perceived detriment to the child (for instance, in the limb-chopping scenario, assume prosthesis are advanced enough to match the abilities of natural limbs). Also assume that the procedure is painless, performed by a certified medical professional, and does not impede the mental and physical development of the child.
Are there any laws that specifically prevent these sorts of modifications? Or are there more general parenting laws that apply?