The poster, which is live now on Tottenham Court Road, London, has a collection tin attached.
When the public drops money in, an film of the girl in the recent "Turn Around" TV campaign gives them a message.
Nick Gill, the executive creative director of BBH, said: "This poster will make people look twice, and won't fail to evoke emotion. The girl doesn't move much, you can just see her breathing, but when a coin is put into the collecting tin she looks up at you and breaks into a semblance of a smile."
Barnardo's is running its latest campaign to demonstrate how it transforms the lives of the UK's most vulnerable children and teenagers.
There is a particular focus on youngsters who are sexually exploited, following the launch of Barnardo's report Whose Child Now? The children's charity worked with 1,060 children and young people who were victims of sexual exploitation during 2007/08.
Diana Tickell, Barnardo's UK director of communications, said: "Barnardo's is an acknowledged expert in work around sexual exploitation which is why this area was chosen as the focus for our Turn Around campaign.
"We have been instrumental in putting the issue of child sexual exploitation on the political agenda for the past 15 years and we have developed 21 services across the UK."