You're probably looking at a bit into the future, right? I would recommend waiting until 4E comes out and looking at that. It's been updated to what the current generation seems more interested in. If she watches fantasy shows, super beings don't have the "wait until tomorrow" mechanic. Fast action and easy numbers are the way to go. As it stands, it looks to me like 4E will be perfect for anyone to start playing and have fun with.
If not that, I'm in a game right now that uses the Underdark as the setting but uses the New World of Darkness rules as the system. Let me tell you, it's pretty damn simple. I also like how it's open enough to allow just about anything. It's a lot more customizable than D&D, in my opinion. 7 dots total between Intelligence and Academics? Roll 7 dice. How many are 8, 9 or 10? Roll the 10s again. Total? 4 Successes? You know a lot!
Might seem a bit more complicated than roll a d20 and add your numbers, but I could see a kid going "11 + 3 + 5 + 2 - 4?"