Kidding Around

A kid holding a kid

My Grandma Penny doesn't like kids. She's a sweet, loving woman who babysits my little cousins several times a week. But she'll only refer to them as 'children'. Born and raised on a farm, she has a hard time referring to humans as kids. Kids are goats. When did we decide a baby goat was another name for a young person?

kid (n.) 

c.1200, "the young of a goat," from a Scandinavian source such as Old Norse kið "young goat," from Proto-Germanic *kiðjom (cognates: Old High German kizzi,  German kitze, Danish and Swedish kid). Extended meaning of "child" first recorded as slang 1590s, established in informal usage by 1840s. Applied to skillful young thieves and pugilists since at least 1812. Kid stuff "something easy" is from 1913 (The phrase was in use about that time in reference to vaudeville acts or advertisements featuring children, and to children-oriented features in newspapers). Kid glove "a glove made of kidskin leather" is from 1680s; sense of "characterized by wearing kid gloves," therefore "dainty, delicate" is from 1856.

kid (v.) 

"tease playfully," 1839, earlier, in thieves' cant, "to coax, wheedle, hoax" (1811), probably from kid (n.), via notion of "treat as a child, make a kid of." Related: Kidded; kidding.

As you can see from the etymology, kid's earliest meanings were "delicate and easy" and "mischievous and manipulative". We still use them that way today. Kidding around is obnoxious, pointless, and a waste of time, like anything a child might do. If you treat someone "like a kid", it means to be delicate and not take them seriously. Much in the same way that "like a girl" means "like a wimp". Although the #likeagirl campaign might change that soon. I don't see how "like a kid" is any different.
Words fascinate me and I wish I knew more about the story behind this. Why are we kids and not colts, calves, piglets? Did it start out as a term of endearment? I've heard people refer to loved one as "little lamb" or "sweet kitten", but it's not used to the extent of kid.