I think the difference between livestock tags and earrings in a kitten's ears is that livestock tags serve a purpose; to identify and track the animal. Putting holes in a kitten's ears or neck serves no purpose other than the amusement of the owner. I didn't think before about very young children who have their ears pierced before they are old enough to give an opinion. While I don't agree with that, I still don't think it's the same as doing it to an animal. That child will be able to make a choice at some point during their life to keep the piercing or not while the animal never has that choice.