First things first: to begin learning about hacking electronics with the Pi, a good first step is to get the little beast to start interacting with the outside world — lighting LEDs, responding to switches of various types, making alerts via Twitter or SMS.
We've made this tutorial on the basics of sensing and responsing with the Pi, including:
Getting busy with a breadboard and a few simple components is a great way to get kids interested in the potential of the device (in ways that just introducing them to Scratch or Python may not). For example, when I gave a Pi to my niece (who's in her early teens) she was politely interested, but no more than that. She has already done some Scratch programming at school, and regularly uses a PC, a Kindle and a Blackberry. The Pi is just another computer for her — and rather a slow one, at that.
I've had similar experiences with several other people. For those who are lucky enough to have lots of access to computers already, the fact that the Pi delivers a magical amount of oomph for such a cheap package is not enough to really spark their interest.
To try and make things more interesting, we've started some tutorial materials that hook up simple electronics to do things like warning you if your parent is coming up the stairs, or sending you an SMS if the bathroom is flooded, and the like. The first few projects are all about sensing and responding: getting the Pi to interact with the outside world. All you'll need is a basic electronics kit (breadboard, resistors, LEDs, switches, a transistor or two, a buzzer, ...), some spare time and, preferably, no allergy to nerds.