preschooler vs small fry

preschooler and small fry both are nouns.

Word	Noun	Adjective	Verb	Adverb
preschooler	Yes	No	No	No
small fry	Yes	No	No	No

As nouns, small fry is a hypernym of preschooler; that is, small fry is a word with a broader meaning than preschooler:

Other hypernyms of preschooler include child, fry, kid, minor, nestling, nipper, shaver, tiddler, tike, tyke, youngster.

preschooler (noun)	small fry (noun)
a child who attends a preschool or kindergarten	someone who is small and insignificant
	a young person of either sex