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