foster-child vs kid

foster-child and kid both are nouns.

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

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

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

foster-child (noun)	kid (noun)
a child who is raised by foster parents	young goat
	a young person of either sex
	a human offspring (son or daughter) of any age
	soft smooth leather from the hide of a young goat

foster-child (verb)	kid (verb)
	be silly or tease one another
	tell false information to for fun