foster child vs nestling

foster child and nestling both are nouns.

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

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

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

foster child (noun)	nestling (noun)
a child who is raised by foster parents	young bird not yet fledged
	a young person of either sex