Okay.... I haven't read all of the responses, but I've got to chime in here.
We opted to surprise our kids with their first trip to WDW (they were DD at 6.5 and DS at nearly 4). We held the secret in for months and on the day of departure we told them we were going to Boston for the day (not a highly unusual thing for us). Had to find a reason to explain the long drive and the suitcases. When we got to the parking garage at Logan my DH pulled out the video camera and I did the "big reveal". Well.... DD pouted and cried and generally was NOT happy that we "lied" to her and she was royally put out that she wasn't going to spend the day at the aquarium. DS seemed okay at first, but then when we told him we were getting on a plane (his first plane ride) he got very nervous. This was just a few months post 9/11 and even though he was very young, he was aware (to some degree) that something bad had recently happened involving airplanes.
DD was also upset that she was unable to help pack her bag and she was VERY concerned that we may have forgotten something that she would deem VERY IMPORTANT to this trip (like her stuffies and such). In hindsight I can understand that feeling because I would be nervous if someone else packed my bag for an important trip! Plus we pulled them from school for the trip and even as a first grader, she was concerned about that.
In the long run, we did have a great time.... but it wasn't the experience I was hoping it would be and I know that a big part of that was a lack of preparation for the kiddos.
Since this was their first trip to WDW they didn't have ANY IDEA of the scope of things to do and would get very caught up (especially DS who wasn't quite 4 years old) in what we were doing and wouldn't want to leave (he wanted to do the Indy Speedway over and over). I realized it was because he really didn't know how much other wonderful stuff was available!
Had we talked to the kids about the rides and where we were going every day and such, they might have been better prepared and had a better time. And perhaps we could have eliminated some "scary" rides. I made the mistake of taking my son on a ride the VERY FIRST DAY that scared the heck out of him (bad Mom award for that one
) ..... and for the remainder of the trip he was VERY LEARY of riding ANYTHING that he couldn't see. That was one reason why he loved the Speedway and Teacups.... he could SEE what he was getting himself into! We couldn't even get him on Buzz Lightyear's Space Ranger Spin.... and this was at a time when Buzz was his HERO! It was sad and disappointing. And I (of course) blame myself.
Truth be told, many folks tried to talk me out of surprising my kids and I kept poo-pooing them. I wanted that amazing "reveal" video to cherish for a lifetime. Instead, I got a video that my DD was embarrassed to even watch for 10 years and she would get FURIOUS if we showed anyone.
If I could go back in time, I would probably tell my kids about one week in advance, at least, to give them time to process it, time to ask questions, time to maybe see some pictures and some time for that fun ANTICIPATION!
That being said.... our second trip in 2004 we told them at the 100 day mark. That gave them plenty of time.... maybe too much!
Anyhow..... please give some thought to maybe giving your kids at least a FEW days to "look forward" to this big event and maybe ask some questions. You can still do a big reveal.... maybe over dinner one night.
Whatever you decide, I hope the trip is wonderful...............P