The Collaborative International Dictionary of English v.0.48:

Kindergartner \Kin"der*gart`ner\, n.
   One who teaches in a kindergarten.
   [1913 Webster]

WordNet (r) 3.0 (2006):

kindergartner
    n 1: a child who attends a preschool or kindergarten [syn:
         preschooler, kindergartner, kindergartener]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "kindergartner":
   classmate, fellow student, grade schooler, high schooler, infant,
   intermediate schooler, nursery school child, prep schooler,
   preschool child, preschooler, primary schooler, school child,
   school kid, school lad, schoolboy, schoolfellow, schoolgirl,
   schoolmate, secondary schooler