When my son was four years old, I bought him a Canon Sure Shot Owl 35mm camera. This was a simple to use point and shoot 35mm camera with a large viewfinder that was easy for a little kid to see through.
A year earlier, he had gotten interested in touching and looking through my expensive professional cameras, so I bought him a single use Fuji Quicksnap camera. At age three, he quickly learned how to turn the flash on and off, how to compose the photo and advance the film. He ended up going through a bunch of those Quicksnap cameras, which got expensive! They cost about $6 each at the time, when one could buy a four-pack of 35mm color film for $7.
For his fourth birthday, I bought him the Canon Owl. Everyone said he was too young for a camera like that, but he loved it! He learned to load and unload the film himself and even learned how to use the self-timer to do self-portraits. Today, at age 15, he shoots with a Nikon digital SLR, using it to photograph scenes that he builds with Lego bricks. He uses the photos to make stop-motion animations! You can see some photos of him working in his 'studio', using a lamp to light the scene.
These photographs were made soon after I gave him the camera, a few months after his birthday (click the Related Photographs tab above the picture to see the rest of this series). In the first photograph, he is photographing me. In the last, he looked bored because I asked him to hold still so I could do a portrait of him sitting there with his camera. He still gets annoyed sometimes when I take too many pictures!
See some of the photographs that Mack made when he was a little kid