small fry vs nestling

small fry and nestling both are nouns.

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

As nouns, small fry and nestling are synonyms defined as:

Other synonyms of small fry include child, fry, kid, minor, nipper, shaver, tiddler, tike, tyke, youngster.

small fry (noun)	nestling (noun)
someone who is small and insignificant	young bird not yet fledged
a young person of either sex	a young person of either sex