urchin vs small fry

urchin and small fry both are nouns.

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

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

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

urchin (noun)	small fry (noun)
poor and often mischievous city child	someone who is small and insignificant
	a young person of either sex