I think it really depends on the personalities of the kids and their ages. Our youngest ds is 13 years old. He has been to Disney twice, when he was much younger. Our oldest ds will be a college sophomore. Our youngest never would have guessed that he was getting to go to Disney again.
It has been so much fun to talk with him about rides and what he wants to do. He has seen the house we will be renting. There is this neat Disney game online and he has been playing it, and we watched the free Disney DVD.
My point is that no matter how old the kids are part of the fun of Disney is the anticipation. I know many might disagree but I would not go with a total on the spot surprise. Instead I would give them at least a few weeks to look forward to their trip, to dream about it, to picture themselves at Disney.
If your children are very little you may only need weeks to do all this "Disney dreaming," but if they are older they will want to be in on the packing,choosing clothes to wear, talking about the rides, watching Disney movies....