child vs poster child

child and poster child both are nouns.

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

As nouns, poster child is a hyponym of child; that is, poster child is a word with a more specific, narrower meaning than child:

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

child (noun)	poster child (noun)
a young person of either sex	a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes
a human offspring (son or daughter) of any age
an immature childish person
a member of a clan or tribe