poster girl vs poster child

poster girl and poster child both are nouns.

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

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

poster girl: a female 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 girl include adult female, woman.

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