We took two nephews this past October, but sprung it on them as one of their Christmas gifts last year. I made up little coupons on the computer with pictures of the castle, Mickey, stars, etc... one for each of them. On it, it had their name and "You're going to Disney World!" - we also put the dates on there and obviously that they were going with Aunt WDWGuru and Uncle WDWGuruDH (and grandma). We put them in envelopes and they opened them with the presents.
We were worried about them knowing so far in advance and dying of anticipation, but it was actually good. Our younger nephew, who had never been (the older one we took a few years ago on his first/only trip), talked about it daily until the day they left! It also let them do chores and save up money to take, plus we gave them Disney Dollars for other occasions in between so they had a little stash when they went. You obviously won't have long to wait!