I don't know....look at it this way - once all your kids know the 'truth' - you don't have to worry about the different wrapping paper, accidentally leaving pricetags on, having them actually think they might get that REAL puppy or pony....it kind of makes the whole thing easier so I'm sort of looking forward to that day.
I'm in the camp when people I know have old(er) kids who claim to believe that they're just pretending for mom and dad's sake (and the moms and dads I know are grateful they're pretending) - I think it's right around the time they realize the guy/girl in the Mickey suit is NOT Mickey that they figure out the truth about the guy in red too. So by admitting it to you - at least you don't have to keep as much of the charade (although part of the charade is fun so I'm sure you'll keep it up) instead of her keeping quiet for the next 4 - 5 years.
I was very young when I discovered the truth (I have older sibs - enough said) - so I was maybe 5-ish and that first Christmas was tough - but after that it kind of got fun - helping my mom find 'santa' gifts for others and knowing how much work she was putting into getting that tree surrounded by gifts every Christmas morning.
So there IS a bright side....