The amount is hard to determine. I usually encourage my kids to bring their own allowance Xmas/Bday money, and then we give them $50 each "Disney money" for our typical DLR trips, and it was $100 for our two weeks in Orlando over the summer. We cover all meals, the occasional treats, and usually buy each kid something fun, like a T-shirt, some pins, or another collectable. Their money is Disney money because if they don't use it in the parks, it comes back to Mom and Dad. This prevents them from wheedling us with sweet smiles and extra hugs for one more churro or another ice cream sandwich, all the while planning to buy a new video game when they get home!

It's not that they are manipulative or that they try to be sweeter than usual, it just happens on vacation. Of course, it might just be me, but I find that it is very easy to give them more than they need once I am in the parks, so this way we all know how it works up front.
A word about the prepaid Visas; we used to use these for every trip. That way, when the kids were younger, we didn't have to worry about them keeping track of bills and change. What we discovered, however, is that there are some drawbacks. There are fees, the money can "expire," and they are not accepted everywhere. Just as a heads up.
