Cyanotypes are a fun, simple project. They teach kids about the concept of light leaving an impression on a surface, as well as the basic safety and carefulness that they should employ when working with chemicals. It's a process which has a high success rate, which is important while working with children.
[Note: For all ages, I am an advocate for using the chemicals to coat your own paper, as opposed to getting pretreated nature print paper. There is a greater appreciation for the final image and the process that is taking place when one coats their own paper. ]
This process has been around since 1842. It was used by Anna Atkins (arguably the first woman photographer) to publish what is often considered the first book to use photographic images as illustrations. It was also used for a longtime to make blueprints.