poster child vs nipper

poster child and nipper both are nouns.

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

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

poster child: a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes
nipper: a young person of either sex

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

poster child (noun)	nipper (noun)
a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes	a grasping structure on the limb of a crustacean or other arthropods
	a young person of either sex