foster child vs nipper

foster child and nipper both are nouns.

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

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

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

foster child (noun)	nipper (noun)
a child who is raised by foster parents	a grasping structure on the limb of a crustacean or other arthropods
	a young person of either sex