street arab vs street urchin

street arab and street urchin both are nouns.

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

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

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

Other hypernyms of street arab include guttersnipe.

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