street urchin vs gamin

street urchin and gamin both are nouns.

Word	Noun	Adjective	Verb	Adverb
street urchin	Yes	No	No	No
gamin	Yes	No	No	No

As nouns, gamin is a hyponym of street urchin; that is, gamin is a word with a more specific, narrower meaning than street urchin:

street urchin: a child who spends most of his time in the streets especially in slum areas
gamin: (sometimes offensive) a homeless boy who has been abandoned and roams the streets

Other hyponyms of street urchin include gamine, street arab, throwaway.

street urchin (noun)	gamin (noun)
a child who spends most of his time in the streets especially in slum areas	(sometimes offensive) a homeless boy who has been abandoned and roams the streets