According to Fundación ANAR (Aid to Children and Adolescents at Risk Fundation) the poster gives children -- the potential victims -- a different message than it displays to adults.
"Knowing the average height for adults and children under 10,GREY has created two different messages," reads the video's description on YouTube. "Using an outdoor lenticular we show adults an awareness message, while children see a message where we offer them our help and show them the telephone number. A message only for children."
Basically, while adults see this message:
Children see this message:
A video explaining the ad was uploaded on YouTube on April 24 and has been viewed a little under four million times in two weeks.