Well, I have to disagree with the previous posters, although it does depend on the ages of your kids. We're going in 2 weeks but told our kids when we booked in October. It has been so wonderful planning the trip together as a family. This is our first big vacation in 6 years and my husband had just ended a long stretch of unemployment, so we've been celebrating together that we can go on this major vacation.
My kids are 8, 5, and 2 and we allowed each of them to choose a restaurant before we made ADRs. We also watching the planning DVD together, read guide books, etc. It has just been so exciting planning this trip with them. I feel like I would have been cheating them out of fun and anticipation if we didn't tell them. It would be different if we went to Disney World every year, but we're an every 4 to 6 years family.
However, it has been difficult for the 2 year old to wait this long (EVERY DAY she's been asking, "we go Disney World right now?") So I would be more likely to surprise/wait if they are younger or if you go to Disney frequently, and more likely to tell them if they are older or if this is a rare event for your family.
Have FUN either way!!!