Short answer: When made to kids who can't decide or defend themselves it's abusive, barbaric, ignorant and cruel.
It's not just cosmetic as you say, it's a mutilation of a part of the body that nature created over millions of years of evolution, it's SUPPOSED to be there, but I guess nature did not take into account human ignorance also known as human culture/religion.
If anyone wish to do it, fine, do it to YOURSELVES, it's your bodies but not on a kid who has no say in this matter.
The hygiene excuse is total BS, true that you are cleaner with it cut but by the same logic you'd be cleaner if your nails were all pulled off at birth, there would never be dirt under them, same with your arm pits, they would be automatically cleaner if you cut off your arms. The solution is simple, teach your kid to be clean down there, to wash it all well when in the shower instead of using the lazy alternative of barbaric mutilation.
Please don't do that to an innocent child.
He who has nothing to hide does not deserve to be called an individual.