Check out this video for an ad about children abuse. It has a secret message hidden to adults that is visible only to children. The trick is its showing different messages at varying eye levels. The reason I love it, is its honest and doesn't talk down to kids, but empowers them! The message they see is a young boy with physical signs of abuse and a hotline number to call if they are in danger themselves.
Furthermore, its interesting to think of how this technique could be used in other advertisements, for better or for worse.