What a great surprise, disneyag! My kids love that kind of surprise.
Last time we went to WDW, we told the kids we were going, but not when. My DH picked up our then 3rd grader from school at lunch time with a note from me saying that she wouldn't be going back after lunch, that we had an airplane to catch. The teacher knew about it for a month and was great at keeping the secret. The other kids at school thought it was awesome too. In fact one day this year I picked up our DD from school early and one of her friends asked if we were going to Disney World AGAIN!
This time I asked if they wanted to know when we were going, they all decided they want to be surprised again. This helps in packing, so I don't have to do it secretly.