Ask HN: Resources for teaching kids robotics?

I was asked by my kids' small, private school to teach either a robotics elective to the Jr. High kids (grades 7 & 8) or a club (for grades 3 - 8) next semester.

From what I understand, it's too late to get involved in the Lego robotics leagues. I've done plenty of Arduino hacking so I'm leaning towards that route.

Anyone have recommendations for kits/tutorials that would be good for that age?