My SD is 9yrs old.
My nine year old found out last year. He repeatedly asked questions about him and was very concerned that he be told the truth. We talked about it and I told him that there is a magic to Christmas whether you are the one experiencing it (those who still believe) or part of keeping that magic alive for others. And that magic is what I think of when I say I believe in Santa. It is magically that people care enough to find special gifts and treats to share with those they love. There is great joy in watching a little kid wake up and see the gifts Santa left both in the stockings and under the tree. There is great joy in finding those little stocking gifts that will delight and then sneaking them into the stocking without being caught. This year he has been helping to keep his 5 year old sister's dreams and wishes about Christmas going. He has written letters with her. They have been discussing how Santa does different things (get down the chimney) They came up with the idea that Santa needs to pay for raw materials and he needs to pay the elves as well as tons of other bills to keep the factory running. Lindsay has decided that Santa buys things like electronic games and toys because the elves just can't do a good job on those types of toys. So Santa is on a budget just like everyone else and that is why he can not always get you everything you want on your list. The answer "it's magic" only goes so far apparently. Part of these ideas have come from attempts to answer concerns over the giving trees at the mall and her daddy's work (how come Santa doesn't just get them everything if their parents can't afford to?) Part are because they hear people talking about how tight money is this year, how expensive everything is and overhearing someone say that Santa has to get in line for the Wii just like everyone else.
So my advice would be to "draft" her. Get her to start helping to keep the dream alive for her younger siblings. That once you reach a certain age you get to become part of Santa's helpers. You become part of the magic rather than just getting to experience it. Being one of Santa's helpers means that you are trusted to help keep the magic alive. Then you come up with a new tradition that is just hers and yours. Maybe she gets to go shopping for the stocking stuffers with you. Maybe she gets to stay up just a little bit longer (or you go in and get her up after the younger kids have fallen asleep) and help put the stockings together and eat the Santa cookies and drink the milk/hot cocoa. And don't forget to get her something special and "grown-up" that is from Santa. Basically you are letting her see that there are benefits of keeping that magic alive for those who still believe and she isn't going to lose out because she now knows.