Consider some research related to the history and playing techniques of the cajon. There are some interesting websites available to support this.
Here is a useful video related to the history:
Useful text and images
Cajon Groove Guide also gives some useful tutorials that you might like to try with your students if you have canons on hand!
And some useful ideas for making your own!
Re: Cajon research
I've been gathering more info about the drum so let me share one site with you, I found it in the search for the best cajon for me:http://cajonguide.com there is not much info on this insturment around the web, but when someone want to find something, normally it the end it succeed.
Do you happen to know any decent brand that produces kids cajons?