Here are five words commonly associated with children that at one time meant something a little different than what they mean today:
naughty - If you read the King James Bible in the book of Jeremiah, you may come across a verse about a basket of "very naughty figs." How, you may wonder, can figs be naughty? The word naughty came from the word naught, which means "nothing." To call something naughty thus meant it was "good for nothing" or "worthless." Now we associate the word with bad behavior, even when it's done by those little darlings we value above all else.
spank - This word once meant "to move rapidly." A spanker was a person with a quick, lively step. To be spanky was to be lively or frisky. Somehow spank came to mean slapping a child's bottom, perhaps because the slaps were usually delivered rapidly.
tantrum - More than a century ago, a tantrum was a foolish fancy or whim. Today we use the word to describe a child's uncontrollable crying fit.
truant - Go back 500 years or so and a truant was a common beggar, not a child skipping school.
waif -- A pursued thief will sometimes throw away whatever stolen goods he is carrying. This discarded property was once known as a waif. Later the word came to be applied to an abandoned or lost child.