I have done the cyanatype sun kits with my kids when they were about 4 and the look on their face was priceless. The remainder of the kit does not last long once the first print is done. They are now 7 and 5 years old asking to make pictures in the dark room rather than watch TV. A nice problem to have.
I feel the key is to show the young children these processes in a fun way, and they will never forget the moment.