I forgot what it's called but I've read that it's a later development. The reason I remember is that Walmart had a t-shirt that had a collage whose edge clearly made a drum set. My son has and loves his rockstar drumset. I said Look at that shirt! Look what it is!! And he started telling me all the pictures inside the collage. If he had seen the drumset he would have been excited and REALLY wanted that shirt. That's when I remembered that I had recently read that little kids couldn't see stuff like that. I can't remember what that's called though.
I can't remember, but here's some fun optical illusions I found on google while trying to remember.
http://slodive.com/inspiration/optical-illusions-for-kids/
