I would plan on a whole day. The first time we went, we were there for 7 hours. That was with two boys, ages 8 and 10. They even wanted to go back another day instead of going to a park.
As for bringing extra money, you really only need it for food. There used to be games that cost extra because they gave away tickets. You could then redeem the tickets for the cheapy little stuff like they have at Chuck E. Cheese. I'm not positive, but I think I read that they have done away with that. There's one area where you can make your own crazy toy from "Toy Story." You can look at it on the computer screen, and decide if you want to buy it or not. If so, they assemble it and sell it to you. I can't remember how much it costs, but maybe around $10. You can also make your own CD and buy that, I think. We've never done it so I'm not sure on this one.
DisneyQuest is a blast. You'll love it!