The Free Dictionary says:
poster child - a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes; "she was the poster child for muscular dystrophy".
However, the term is usually used metaphorically, meaning a shining example or model of its type.
"Poster child" seems to be one of those Americanisms that spontaneously arise from time to time, whose meanings everyone is supposed to know intuitively, but which are opaque to those from other cultures.