9 and 10 year olds.
This idea has been borrowed from http://afaithfulattempt.blogspot.co.nz/2011_07_01_archive.html. Many thanks. The class enjoyed the challenge. They were younger, but the concept was still easily understood by them.
Check out "A faithful attempt" blog as her instructions are very straightforward. My students only had a morning to complete this project which was only just enough time.