poster child

noun

Etymology: From the usage of appealing young people and children in charitable advertisements.

1. One who is a prototypical or quintessential example of something.

   “He's a poster child for militant vegetarianism.”

   “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.””