Green’s Dictionary of Slang

a child, a small person.

1975		J. Wambaugh Choirboys (1976) 283: ‘Hey, munchkin!’ yelled Roscoe Rules [...] ‘You’re the littlest guy around here.’.
1983		R. Price Breaks 65: Guarding the bag of little Snickers bars for the munchkins that rang our bell.
1990		S. Morgan Homeboy 13: A special munchkin was Rigoletto, twice a freak by virtue of a monstrous member.
2000		Indep. Rev. 4 Feb. 15: Som has berated the harmless indie munchkins that make up [...] his audience for lack of perceived enthusiasm.
2014		J. Porter ‘Good Luck in Puerto del Oro’ in ThugLit July [ebook] ‘The truth about what? The uncontrollable urge to punch the munchkin?’.