small fry vs juvenile

small fry and juvenile both are nouns.

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

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

Other hypernyms of small fry include juvenile person.

small fry (noun)	juvenile (noun)
someone who is small and insignificant	a young person, not fully developed
a young person of either sex

small fry (adjective)	juvenile (adjective)
	of or relating to or characteristic of or appropriate for children or young people
	displaying or suggesting a lack of maturity