The Collaborative International Dictionary of English v.0.48:

Kilderkin \Kil"der*kin\, n. [OD. kindeken, kinneken, a small
   barrel, orig., a little child, fr. kind child; akin to G.
   kind, and to E. kin.]
   A small barrel; an old liquid measure containing eighteen
   English beer gallons, or nearly twenty-two gallons, United
   States measure. [Written also kinderkin.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

kilderkin
    n 1: an obsolete British unit of capacity equal to 18 Imperial
         gallons

Bouvier's Law Dictionary, Revised 6th Ed (1856):

KILDERKIN. A measure of capacity equal to eighteen gallons. See Measure.