Do you believe a child should have the power to decide what happens to their body even at a young age? What if a 6 year old wants to get a permanent tattoo? Or a middle schooler wants a subcutaneous horn implanted on the forehead to look cool? Or a toddler refuses medical treatment? I doubt that's what you mean.

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