Don't under estimate the thought process of young kids. They might not know ehat they are exactly doing with the camera, but they do graps the "action-reaction" thought behind it.
My son is 2 years and he frequently shoots with one of these disposable Fuji things, most shots are not good, some however are pretty well framed and he grasps the idea that he took them. It takes time and loads of rolls but at one stage they do see it, and that at very early age.
Kids are basically clever, they don't often get things right but occasionally they do, and they know how they did it....
I would say keep on trying.... We have now lots of pictures of our pets, my sons friends (cuddly toys and stuff) all taken by him, and they hang on his bedroom wall, and he loves them.
On average he shoots 1 to 2 rolls a week, and doing it with these throw away thing it cannot cause much damage.
It will be a couple of years however before I hand him over his Leica Mp with 50 summicron which I bought on the day that he was born (all paperwork dated on his day of birth) and hasn't been used yet. He will be the first to shoot a picture on that one.
I#me not pressing him to become a photographer, I would just like to share my hobby with him.....