Actually, my brother gave up before even trying (at this age "passions" are sometimes a very passing phenomenon).
I'd recommend pygame over web development. Making a video game is much more fulfilling at that age in my opinion. Ask any 12 year old "do you want to make a video game or a website?"
For an older kid (~16), web development with APIs such as Facebook can be interesting too, because (s)he'll want to brag about it to his/her friends.
I really like the Facebook app/games idea, since current video games set the bar way too high for any individual working alone. Making "Pong"-type of games will probably not impress anyone these days :)
Got a lot of great answers.