poster child vs tiddler

poster child and tiddler both are nouns.

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

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

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

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