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.
Poster child does not need to be in quotation marks, and there is no reason to hyphenate it.
Examples
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 environmentally friendly accessorizing. [TampaBay.com]
The poster child of overdiagnosis is prostate cancer screening. [Los Angeles Times]