The property of the person who inhabits them, and no one else's. And that as such, parents' power of decision-making should be limited to procedures that are medically necessary to protect a child's life and health, and things like cosmetic body modifications should be left to the mature individual's own prerogative.
The parents aren't doing this alone. They have the support of society and the pediatrician involved. But worse, "medically necessary" isn't as clear a term as we might like it to be. Baby in an irreversible coma? "Medically necessary" would imply you can't unplug them.
The problem remains. If the touchstone is going to be one of choice, and we have a human who cannot choose (same for the mentally incapacitated, demented, senile), then the buck has to stop somewhere. I prefer it stop with the parent in this case, even when the decision is not one I would personally choose.
I am relying on an assumption that parents really want to care for their children as best they are able, as well as monitoring by a medical professional. However, judging by a circumcision rate over 70% in the US, it appears that my assumption is not shared by others here. Because those circumcisions were approved by parents, and if circumcision is wrong, then those parents are at least partially to blame and not so caring as I think they are.
And I think the default should be "prove it's harmful," not, "prove it's helpful." With the follow up of, "Is it harmful enough to interfere?"
Furthermore, the concept of "hands off my body" isn't well thought through, especially in the case where the body in question isn't owned by a reasoning being. This same thinking has led to a disastrous situation when it comes to organ donation, where the "owner" has to be consulted before their organs can be harvested to save another's life.
And finally, the idea that "evolution knows best" is hogwash. Evolution doesn't know anything at all. Nature is our enemy, not our friend or our God.
I can hardly wait for the kerfuffle that will ensue once in utero genetic modification is available in a big way. Sure, we'll start with fixes to known congenital defects, but customization with a nod to cultural norms won't be far behind. At least we still think the fetus is "owned" by mom.