I don't know what kids are generally interested in... Maybe robotics? You could start with sometime "simple" related to servo motors, but there are some complicated things going-on "behind the scenes" in the servo library.
And then it depends on what you want them to learn... Some programming? Some electronics? The core Arduino C++ language is simple enough that I think it makes a GREAT introduction to programming.
But you do have to learn some electronics at the same time and that complicates things. Before you study microcontrollers in college you'd have a couple of years of electronics & programming under your belt. But, most people using the Arduino are self-taught hobbyists so all of the prerequisites aren't required.