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