When Brian asked me to teach his seven year old son computer programming, I was reminded of that question people asked on slashdot: what materials to use for teaching computer programming to kids?
While teaching Brian C++, using pretty helpful online resources, I have seen a few interesting suggestions for kids. Arduino would be fun (I'm learning myself), I would go with LOGO, since I'm more familiar with it, as it was my childhood course my parents sent me to as well.
So we use MSWLogo. Though I've heard there are plenty of clones out there.
And it is good to collect projects and course materials for these.
1. http://www.educa.fmf.uni-lj.si/logo/doc/Apr.96/chpt4.pdf (For color and sound).
http://www.cprogramming.com/tutorial/c++-tutorial.html
Now another, CodeSpells, currently running under MacOS. It is a video game promoting Java programming.