"What's up?" and " Not much?" come from African-American hoods... Teaching such phrases to kids is not appropriate, I believe. They will sound "Black". I am 46 years old. When I came up, we, African-Americans, were the only ones using such phrases.This is a variation of the recency illusion. In fact, the phrase is well over a hundred years old and can be found all over the English speaking world.
From the 1858 novel Paved with gold (based in London, England):
"The little square wicket in the gate was opened and a round red face appeared behind the gridiron like bars. The eyes of the face twinkled again as they glanced at the prison arms on the warder's stand up collar and the mouth was seen to expand into a grin as its owner said: 'Now then what's up? You ain't come after any of our chaps hare you?'"From the 1870 British Flac and Christian Sentinel:
"A Dialogue in a Barrack room an Old and Young SoldierIt's also cited in Austral English: a dictionary of Australasian words, phrases and usages with those aboriginal-Australian and Maori words which have become incorporated in the language, and the commoner scientific words that have had their origin in Australasia (1898) on p. 445.
William: Good morning Tom you appear be in a great bustle What's up?
Tom: What's up? Not much. I am putting my traps together as I am going on furlough to-day."
Before you go claiming (or disparaging) an innovation, it's best to check the facts.