I had my foreskin removed when I was a child. It was against my will. It was medically unnecessary. It was harmful in that it denied me my foreskin. It was removed before my foreskin had separated from my glans, so it was done by cutting the foreskin itself and then tearing it away from the glans, not only denying me a foreskin, but also damaging my glans. This wasn't a side-effect. It was normal way that the procedure is performed in a hospital. Again, to no medical benefit.
I had a body part removed against my will. My parents both would make a different decision today if they'd had better information. However, it needn't have been their decision. The decision could have been made by me when I was old enough or by them if it became medically urgent. Neither of these things happened.
I don't blame my parents for not knowing better, so this isn't about being angry at them. However, if you claim that having a healthy, normal body part removed from me is not harm, then I'll suggest a place where we can meet and I can choose any number of body parts I don't find necessary and remove them. Don't worry, I promise to take every reasonable measure to deal with your pain that was taken during my circumcision. You let me if you life it I committed assault.
The requirement is not that I prove that there is some medical harm (beyond the obvious harm of removing a healthy body part from a child). If you want to conduct an unnecessary cosmetic surgery on a child that irreparably removes a part of their body, you're going to need a better reason than "well, it might have at one time had a minor impact on totally preventable issues" or "it might prevent a disease you'll only get when you're old enough to make a decision yourself" or "it might help with an unlikely disorder that can be solved if it ever occurs".
See, my foreskin wasn't removed because I'm Jewish. It was removed because we made it a political issue in the US and politically attacked pediatric associations for recommending the ending the practice (as a prevention method). However, my great uncles and all the men before that generation had it removed as a religious practice and, frankly, that is no better. We consider FIRST whether one has a right to bodily autonomy and THEN respect religious rights and practices. I, personally, believe that I deserved the right to grow up intact except where medically unavoidable. My parents were meant to be surrogates for me during a time when I was not old enough to make decisions for myself. They were meant to make decisions for me that were necessary to get me safely and responsibly to adulthood. I was not their slave and they did not have a right to conduct whatever random rituals they like to my body.
If a religious reason is adequate then any reason should be adequate. Why shouldn't I be allowed to tattoo the faces of my children or cut of their earlobes or simply do whatever process of scarring or cutting their body I choose so long as it's not more dangerous than circumcision and so long as it doesn't prevent the use of a "vital" body part as defined by circumcision advocates. So advocates, do I have the right to tattoo the faces of my children? Do I have the right to cut off their earlobes? Can I get liposuction for my toddlers? What makes cutting off a part of the penis so special?
