The phrase poster child originally referred to a child who
appears on a poster for a charitable organization. For example,
a poster child might be a child cancer survivor on a poster of
an organization that fights cancer. While this older definition
remains, a new, idiomatic meaning—a person who is a prominent
example or representative of something—now appears more often.
It’s even used in reference to people and things that are not
children. For example, these writers use the newer sense of
poster child: She will be Microsoft’s poster child for getting
teenage girls interested in technology. [Montreal Gazette
(article now offline)] There are some ambitiously big plans in
the pipeline for little Lydia Ko, New Zealand golf’s prized
poster child. [New Zealand Herald] Rolando Villazon is the
poster child for a tenor who quickly grabbed the limelight and
as quickly fell. [Washington Post] Poster child sometimes even
appears in reference to inanimate things—for example: Bamboo is
the poster child for environmental