The Jargon File (version 4.4.7, 29 Dec 2003):

munchkin
 /muhnch'kin/, n.

    [from the squeaky-voiced little people in L. Frank Baum's The Wizard of Oz]
    A teenage-or-younger micro enthusiast hacking BASIC or something else
    equally constricted. A term of mild derision ? munchkins are annoying but
    some grow up to be hackers after passing through a larval stage. The term
    urchin is also used. See also wannabee, bitty box.


The Free On-line Dictionary of Computing (30 December 2018):

munchkin
urchin

   /muhnch'kin/ [Squeaky-voiced little people in L. Frank Baum's
   "The Wizard of Oz"] A teenage-or-younger micro enthusiast
   hacking BASIC or something else equally constricted.  A term
   of mild derision - munchkins are annoying but some grow up
   to be hackers after passing through a larval stage.  The
   term urchin is also used.  See also wannabee, bitty box.

   [Jargon File]