foster child vs youngster

foster child and youngster both are nouns.

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

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

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

foster child (noun)	youngster (noun)
a child who is raised by foster parents	a young person of either sex