WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "preschooler":
   babe, baby, baby bunting, bambino, classmate, fellow student,
   grade schooler, high schooler, incubator baby, infant,
   intermediate schooler, kindergartner, little angel, little darling,
   mewling infant, neonate, nursery school child, nursling, papoose,
   preemie, premature baby, prep schooler, preschool child,
   primary schooler, puling infant, school child, school kid,
   school lad, schoolboy, schoolfellow, schoolgirl, schoolmate,
   secondary schooler, suckling, toddler, weanling, yearling