street child vs small fry

street child and small fry both are nouns.

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

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

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

street child (noun)	small fry (noun)
a homeless child especially one forsaken or orphaned	someone who is small and insignificant
	a young person of either sex