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

toy problem
 n.

    [AI] A deliberately oversimplified case of a challenging problem used to
    investigate, prototype, or test algorithms for a real problem. Sometimes
    used pejoratively. See also gedanken, toy program.


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

toy problem

   [AI] A deliberately oversimplified case of a challenging
   problem used to investigate, prototype, or test algorithms for
   a real problem.  Sometimes used pejoratively.  See also
   gedanken, toy program.

   [Jargon File]