In teaching programming, I've found that often the hardest thing to learn is how to break down problems logically, not how to write code. For example, if I assign students to write a function that takes an average (mean) of a series of numbers, very few will fail because of syntax errors. Many more will fail because they've never thought about how to take an average!

Therefore, my latest thinking is that the first few programming lessons should be about basic mathematical problem solving. How to compute an average. How to estimate a square root. That sort of thing in a "math" class is assumed given. To a mathematician, the square root of two is "the square root of two" and no computation is necessary. I want to convey to my students that computers have to be taught a process, not an answer.

I'll find out next semester if that's a better approach. I hope you find it helpful or at least interesting.

Something else I loved was that many students got to apply their math knowledge naturally and in a fun way, e.g. shooting bullets at angles, rotating turrets, following parabolic paths, and even a ray caster. (As an added bonus, Processing uses a very similar IDE to Arduino.)

I happened to teach another group of kids Arduino, which was fun but frustrating to some. Our final project of the week was very complicated, and only a few students finished it. I also unfortunately had many students fry parts and boards, which was frustrating when I gave so many reminders how to avoid it. (Though, young students are not exactly careful about double-checking their wiring.)

My recommendation to you for middle schoolers (ages 11-14) is to use Lego Mindstorms. Largely, the middle schoolers in my class were not patient/meticulous enough yet to effectively wire, write software, and most importantly debug when something goes wrong. (After all, it could be the software, wiring, OR a bad part.)