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