My sister, her husband (and I to a lesser extent, since I live in a different state) kept the secret from their then 9 year old sons a little over a year.
We booked reservations for Dec 99/Jan 00 In late November 98..
From what my sister tells me, while watching the Xmas parade on Xmas day 1999, one of the boys actually came out with "I wanna go to Disney World for the Millenium" They thought their next trip back was three years in the future. And the Millenium Celebration was being heavily promoted during the parade broadcast.
They put the boys to bed Xmas night and immediatly starting pulling the tree down to get it out of the house before they left and packing their car (including sneaking into the sleeping boys room to pack their stuff.
They woke them up the next morning "get in the car, we're going to Disney World!" We've got it on video tape
I flew down and arrived in FL the same night they did by car. We stayed in an offsite motel that night and the next morning I met my sister in the lobby while my brother in law gor the boys and their luggage loaded. When we went out to the car I stayed behind my sister as she went up and said something to the boys about picking up a hitchhiker.
It was all pretty exciting.
Deb