The Collaborative International Dictionary of English v.0.48:

Pickaninny \Pick"a*nin`ny\ (p[i^]k"[.a]*n[i^]n`n[y^]), n.; pl.
   Pickaninnies (p[i^]k"[.a]*n[i^]n`n[i^]z). [Cf. Sp.
   peque[~n]o little, young.]
   A small child; especially, a negro or mulatto infant. Now
   (2001) used primarily in the latter sense, and in that sense
   often considered derogatory. [U.S. & West Indies]
   [1913 Webster]

WordNet (r) 3.0 (2006):

pickaninny
    n 1: (ethnic slur) offensive term for a Black child [syn:
         pickaninny, piccaninny, picaninny]

The Devil's Dictionary (1881-1906):

PICKANINNY, n.  The young of the _Procyanthropos_, or _Americanus
dominans_.  It is small, black and charged with political fatalities.