Hey Chris and welcome, from not far South of you.
Answer to your question depends on a few things I guess, not least is do you have some computing equipment to hook them up to? Assuming you do, what level are the kids at already, and what odds and ends did you get with the Arduinos and / or have already.... LEDs, dc motors, servos... any mechanical odds and ends?
There are loads of small things to get started with
here, which might get you thinking about some things to do.