I'm just going to give you my opinion of surprising kids. I have done it once, but it wasn't their first time there. Ironically, that trip was the worst one for my dd. She seemed to have a lot of meltdowns and temper tantrums. Other times she had been fine. I attribute it to lack of acceptance (getting excited for, looking forward to, etc...).
I don't think I would surprise kids on my first trip. There is SO much planning, dreaming, thinking, etc.... that the kids do beforehand that makes the trip that much more fun. They can watch the planning video (free) so they know what things are when they see them. We do fun things like countdown chains, mickey pancakes on Sundays, Disney movie nights, Disney Trivia Pursuit all counting down til our trip. I just think it might be too much of a shock to the older ones especially that they would actually lose out on the experience.
JMHO... I would tell them 30 days out, get them a copy of Disney for Kids (so they can read about all the rides and such), the planning video to watch and a journal for them to write down what rides they want to go on. Also include a paper chain for 30 days and start the countdown then! It's still a surprise but they do have the anticipation before hand which builds the fun when they finally arrive!
Good Luck...
Karen