Haha that was pretty awesome. Here's my explanation:
You're seeing the adult's face almost perfectly placed behind the child's face, so the perspective makes it look like it's the child that has an adult's face, but if you look at the face, you will see two varying skin tones one behind the other (the nose is the adult).
THE OBVIOUS
-There's an adult holding a child in the water.
-There are two arms and two hands in the picture, the arms on the upper body are the child.
-The hands shown are the adult's hands.
-The two bodies have similar flesh tones.
THE NOT-SO-OBVIOUS
-the child is wearing the hat and shades.
-there 'might' be a blurring photoshop between the adult's nose and the child's face, but it's cleverly done to look like white sunscreen or something. It's right by the shades. Or it's natural image blurriness but it's key to masking this issue.
-the nose is the adult's, while the ear is the child's.
Nice time-waster though! :D