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