street urchin vs urchin

street urchin and urchin both are nouns.

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

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

street urchin: a child who spends most of his time in the streets especially in slum areas
urchin: poor and often mischievous city child

street urchin (noun)	urchin (noun)
a child who spends most of his time in the streets especially in slum areas	poor and often mischievous city child