I would probably use these games:
Mario Kart Wii (Or any similar racer): It's a game that I think would be easy to get into. Mario is a well known character, so it wouldn't be completely foreign to them. At the same time, it will get introduce them to single player experience, multiplayer experience, the idea of unlocking things and so on. At this point in time, it's a rather closed experience, nothing is open to go exploring.
Super Mario Galaxy (Or any similar platformer): Sticking with the new games still. I think the platformer would be the best genre to move to next. There's a fine display of graphics here, it's really fun, there's things to collect in order to unlock things. It also introduces the idea of story within a game, although simplistic, I think it's a good idea to start with stories like this.
Bioshock (Or any similar First Person Shooter): I haven't played this myself, except in demo form, but from what I have heard and played it's good for plot and gameplay, whilst it also shows off graphical excellence. This would take the take the player to a more mature idea of gaming - getting rid of the idea that games are only simple and for children, and offering them some plot and tactics.
Final Fantasy IX/Final Fantasy VIII (Or any similar RPG): Starting to take things alot further now. These two games I think would show them, that you can get alot out of a game, and that a lot of effort goes into them. There's the story, the characters, the graphics, the music etc. Although I'm taking them back to the PS1 generation, I think the FMV scenes would ease them into these sort of games, and perhaps wouldn't mind the graphics once they start playing and getting into it more. I'd like them to get the idea, that you're not always restricted, and you can go and explore, battle, play card games, ride chocobos when you want etc.
Maybe end with a sand box game like Oblivion or Grand Theft Auto.
I've only got four games here which I discussed in a little depth. And I'm not sure if it would work. But I thought that would be the general kind of order I would do things. Maybe it'd be better to show them a few more game of each genre, but that's what I came up with by sticking to the rules of the thread.