The oomlout kit mentioned is the best place to start for you. You probably won't need one of those for every kid though, the guides that come with it are on their website and once you know what activities you want to teach you can just buy the components required. The Arduino to start with is the Arduino Uno (£22, $30)

Have a look at the (well illustrated!) examples provided with the Arduino software too: http://arduino.cc/en/Tutorial/HomePage