The Collaborative International Dictionary of English v.0.48:

Playhouse \Play"house`\, n. [AS. plegh[=u]s.]
   1. A building used for dramatic exhibitions; a theater.
      --Shak.
      [1913 Webster]

   2. A house for children to play in; a toyhouse.
      [1913 Webster]

WordNet (r) 3.0 (2006):

playhouse
    n 1: plaything consisting of a small model of a house that
         children can play inside of [syn: playhouse, wendy
         house]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "playhouse":
   Elizabethan theater, Globe Theatre, Greek theater, amphitheater,
   arena theater, auditorium, cabaret, circle theater, club,
   concert hall, cubby, cubbyhole, doghouse, hall, hole, house,
   little theater, music hall, night spot, nightclub, opera,
   opera house, outdoor theater, pigeonhole, pinch, showboat, theater,
   theater-in-the-round, theatron, tight spot, tight squeeze