foster-child vs child

foster-child and child both are nouns.

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

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

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

foster-child (noun)	child (noun)
a child who is raised by foster parents	a young person of either sex
	a human offspring (son or daughter) of any age
	an immature childish person
	a member of a clan or tribe