Maybe you mean a parent and child should decide together? I don't think a child has the capacity to make, or even weigh in on, decisions. Not just a newborn, even older children (most anyway) would have a difficult time making big decisions like if they should have a body part altered by surgical operation. Is it your position to postpone all non-emergency decisions regarding bodily alterations until the person is old enough to decide for themselves what happens to their body? If so, that doesn't seem practical. If you mean something else, please go on
But that is neither here nor there about this discussion, which is someone making an alteration to someone else’s body with zero medical benefit to the someone else.
But the same reasoning would apply. You should not alter someone else’s body for cosmetic reasons without their permission.
If you are assigning religious or cultural value to an act that needlessly mutilates your child’s body, you are missing the principle of freedom and letting people have ownership of their bodies. There are cultures that “value” binding feet, stretch out lips and earlobes, poke holes in various places, and face tattoos.
That has no bearing on the fact that people should not have a cosmetic alteration done to their body without their permission.