What is the origin of the word kid?
Kid is borrowed from the Old Norse word that referred to the offspring of goats. It wasn't until the 1500's that people started to refer to their children as 'kids'. It has since been incorporated into English language as an informal generic name for children. It still also means "young goats".
According to the Online Etymology Dictionary, the origin of the word “kid” dates back to twelve hundred AD, originally meaning, "the young of a goat," from a Scandinavian source such as Old Norse kið, meaning "young goat," from Proto-Germanic kidjom.