poster child

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A child who appears on a poster as a member of a group benefited by a charitable organization.
• noun A person who is a prominent example or type of something.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic One who is a prototypical or quintessential example of something.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a child afflicted by some disease or deformity whose picture is used on posters to raise money for charitable purposes