poster child vs small fry

poster child and small fry both are nouns.

Word	Noun	Adjective	Verb	Adverb
poster child	Yes	No	No	No
small fry	Yes	No	No	No

As nouns, small fry is a hypernym of poster child; that is, small fry 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
small fry: a young person of either sex

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

poster child (noun)	small fry (noun)
a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes	someone who is small and insignificant
	a young person of either sex