well, mama, i have to agree with elk. here's my experience...
i was going to do the same thing and dh and i kept it secret for 8 months! i planned, shopped, planned some more, acting like nothing was up the whole time. we were going to wake them up in the morning and say, 'we're going to disney, the plane leaves in 3 hours!' my kids have both have been to wdw before, 14 yr old 2x and 3 yr old 1x. they know that i'm wild for disney and was planning to go back at the end of this year, so they didn't think anything of the books, internet sites, etc.
on a sunday, we called them into the living room for a 'family meeting' and informed them that we were indeed going to disney-in 4 days!!! i couldn't take it anymore and wanted to share the magic!
my 14 yr old didn't believe us, and sat in pretty much stunned silence for about a half hour, wondering how we had already handled her school and all. my 3 yr old jumped and squealed in joy, then announced that he couldn't go to disney, 'cos that's not my home.'
when they finally got over the shock; my dd understood that the school was aware of the trip and ok with it, and ds knew that he could go on vacation and come back home afterwards, the excitement started. they really got into packing and planning. a couple of thoughts on how this worked out:
-i'm glad we told them, because they apparently needed time to digest such an idea. they gathered all of their important things for traveling that i might not have thought of, and talked about what to ride and where to eat. if we had waited, they might have wasted a day or two there, getting over the shock.
-it added a great deal to my excitement about the trip too! dh is not the disney fanatic that i am and i was excited all alone pretty much. it didn't even seem real to me until the kids new and we could share the fun!
we booked a comeback offer while there and are going back in november, any surprises this time will be special meals, activities, etc. otherwise, the kids are in it with us!
HTH-Lisa