Green’s Dictionary of Slang

munchkin n.

[the Munchkins, diminutive characters who featured in the book by L. Frank Baum, The Wizard of Oz (publ. 1900, filmed 1939)]

a child, a small person.

[US]J. Wambaugh Choirboys (1976) 283: ‘Hey, munchkin!’ yelled Roscoe Rules [...] ‘You’re the littlest guy around here.’.
[US]R. Price Breaks 65: Guarding the bag of little Snickers bars for the munchkins that rang our bell.
[US]S. Morgan Homeboy 13: A special munchkin was Rigoletto, twice a freak by virtue of a monstrous member.
[UK]Indep. Rev. 4 Feb. 15: Som has berated the harmless indie munchkins that make up [...] his audience for lack of perceived enthusiasm.
[US]J. Porter ‘Good Luck in Puerto del Oro’ in ThugLit July [ebook] ‘The truth about what? The uncontrollable urge to punch the munchkin?’.