a poster child for something
a poster child for somethingmainly AMERICAN
If someone is a poster child for a quality, activity, or situation, people associate them with it because they are a very good or clear example of it. Bethany McLean has become the poster child for financial journalists. Note: You can also say that someone is a poster boy for or a poster girl for something. There was a time when it seemed you were a poster girl for late motherhood. He's the poster boy for rugby worldwide. Note: A poster child is literally a young person who appears on a poster advertising something.