In my case, I am the one buying the kits. I have many NXT 2.0 kits and also a nice environment for hardware production with the arduino platform (with everythin

In my case, I am the one buying the kits. I have many NXT 2.0 kits and also a nice environment for hardware production with the arduino platform (with everything even an oscilloscope). Usually, the Legos are used for the younger kids, and if they are more attracted to building stuff than programming then they fork into the open hardware tracks. My advice is to work through projects. Just how real life works, and what the kids find more interesting: there is a problem to solve or an specification on what they want to build, and from that you and them sit down, organize your thoughts, make the design, and then build it, test it, improve it. They love it, it works very well, and keeps you doing very very interesting stuff always with something different. A completely hands on experience, and they learn by doing. As for which arduino kit you would use, anyone will do it. But when they are just starting with arduino, maybe it's better to keep it simple (but always as a part of solving a problem, not doing it to "learn how to turn a LED on"). The best thing is to locate a store and work always with them, so look at what kits they have and choose from there. That's how I do it here in Germany, and worked very well. If you yourself have no experience on arduino or open hardware, I recommend you to choose some book to build little projects from it, so your choice of the kit will be dictated by the kit used on the book. Anyway, congrats for the teaching!