foster child vs small fry

foster child and small fry both are nouns.

Word	Noun	Adjective	Verb	Adverb
foster child	Yes	No	No	No
small fry	Yes	No	No	No

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

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

foster child (noun)	small fry (noun)
a child who is raised by foster parents	someone who is small and insignificant
	a young person of either sex