The Collaborative International Dictionary of English v.0.48:

Childish \Child"ish\ (ch[imac]ld"[i^]sh), a.
   1. Of, pertaining to, befitting, or resembling, a child.
      "Childish innocence." --Macaulay.
      [1913 Webster]

   2. Puerile; trifling; weak.
      [1913 Webster]

            Methinks that simplicity in her countenance is
            rather childish than innocent.        --Addison.
      [1913 Webster]

   Note: Childish, as applied to persons who are grown up, is in
         a disparaging sense; as, a childish temper.
         [1913 Webster]

WordNet (r) 3.0 (2006):

childish
    adj 1: indicating a lack of maturity; "childish tantrums";
           "infantile behavior" [syn: childish, infantile]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "childish":
   arrested, asinine, babish, babyish, backward, beardless, boyish,
   boylike, calflike, childlike, coltish, decrepit, doddering,
   doddery, doited, doting, fatuous, foolish, girlish, girllike,
   immature, inexperienced, infantile, infantine, juvenile, kiddish,
   maiden, maidenly, moronic, naive, puerile, puplike, puppyish,
   puppylike, retarded, senile, silly, simple, slow, sophomoric,
   underdeveloped, undeveloped, unsophisticated, wanting