Is this your first trip?
My kids really enjoy the months of planning and the anticipation. I feel they would have missed out on a lot if we had surprised them. We watched the planning DVD, made the customized maps, and talked daily about Disney. They earned Disney Dollars and we had a countdown chain. We also did Disney movie nights since my little ones hadn't seen most of the movies and I wanted them to be familiar with the characters/stories that have attractions in the parks. They even enjoyed helping with the packing.
When we went last year it was a total surprise for the kids.
We did discuss Disney and made plans for our "next" trip, but told them we couldn't afford to go yet. We told them we were going to visit grandma (south of Disney) and they did not suspect a thing until we actually pulled into the parking lot of the resort. It was fun, but I don't think we would do it again. (We've actually already started planning our next trip and my older DD is even helping me compare the costs of different options and budget how much we need to save each week... A good lesson in math and financial planning.

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Perhaps you could do some sort of big announcement instead of a surprise. I've seen some ideas on this board-- decorating the house and having a special dinner, making T-shirts, etc. That way you'll have the fun of surprising them about the trip, but it will be far enough in advance that they can still participate in the planning and enjoy the anticipation.