I've been working with http://www.snapcircuits.net/ Snap Circuits with my kids since they were 4years old. The lessons they learned there are used to this date and i think invaluable, especially now that they are trying to get things to move and light up using an arduino. I recommend checking out their kits, they can be used all the way up to HS level, heck I still like using them and showing them off to other parents.