Poster Child Meaning

(idiomatic) One who is a prototypical or quintessential example of something.

Example:   He's a poster child for militant vegetarianism.
2005, Susan Cunningham, “Poster Child”, in Unwrapping the Sandwich Generation. Life Vignettes about Seniors & Their Adult Boomer Children[1], Morgan James Publishing, ISBN 9781933596006, page 175:
  I think he smile could have opened the door by itself. It seemed to have a life of it’s[sic] own with snow-white teeth below sparkling blue eyes. Its owner could be the poster child for the expression “grinning from ear to ear.”
2006, Ted Dekker and Bill Bright, Blessed Child[2], Thomas Nelson, ISBN 9780849945137, page 77:
  â€œHe’s not exactly a poster child for the average well-adjusted American kid. He’s only been in-country for a few days.”
2008, Susan B. Neuman, “Changing the Odds though After-School Programs”, in Changing the Odds for Children at Risk: Seven Essentail Principles of Educations Programs that Break the Cycle of Poverty[3], ABC-CLIO, ISBN 9780313362224, page 169:
  A visit to Adventure Island–an after-school program developed by Robert Slavin and Nancy Madden, professors at Johns Hospkins University and creators of Success for All, a comprehensive school reform program practiced in hundreds of schools accross the country–could be the poster child for what some might call the academic approach.