WordNet (r) 3.0 (2006):

trivialize
    v 1: make trivial or insignificant; "Don't trivialize the
         seriousness of the issue!" [syn: trivialize,
         trivialise]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "trivialize":
   beggar, belittle, decry, denigrate, deprecate, depreciate, dismiss,
   disparage, lessen, make light of, minimize, misprize, play down,
   pooh-pooh, put down, run down, scoff at, scorn, slight,
   underestimate, underplay, underrate, undervalue