wonder child vs youngster

wonder child and youngster both are nouns.

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

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

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

wonder child (noun)	youngster (noun)
a prodigy whose talents are recognized at an early age	a young person of either sex