From what I've heard, and that may be wrong, Onion Kid is a japanese term for poor kid. Maybe it was a mistranslation, or maybe the translators thought we wouldn't like the term "poor kid"... Anyway...
Because Onion Kids and onions both contain 'allinases' and 'amino acid sulfoxides'. So when they're cut in half they create a mild volatile gas which irritates your tear ducts.