poster boy vs poster child

poster boy and poster child both are nouns.

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

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

poster boy: a male poster child
poster child: a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes

Other hypernyms of poster boy include adult male, man.

poster boy (noun)	poster child (noun)
a male poster child	a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes