street child vs youngster

street child and youngster both are nouns.

Word	Noun	Adjective	Verb	Adverb
street child	Yes	No	No	No
youngster	Yes	No	No	No

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

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

street child (noun)	youngster (noun)
a homeless child especially one forsaken or orphaned	a young person of either sex