When we went last year, the kids all knew and helped with the planning. (Sadly, our oldest DS wasn't able to get off work to make the trip.) We're planning on going in January, and as of yet, the youngest two (DD11 & DD10) don't know, but our oldest two do. They are much older, and we have to deal with time off of work and around college schedule. As of right now, we're planning on it being their Christmas gift - although we haven't decided how to do that yet. Well, THE gift for the whole family. To add more to the surprise, it will be an extended family vacation - my ILS will be going, as well as my BIL, his wife and my niece who is 7. But we're not telling the kids the others will be there, nor are my ILS telling their daughter that we will be there or that her grandparents will be there. They go every year, so the trip won't be a surprise for her, but everyone being there will be.
My youngest two are constantly talking about wanting to go back, but this time making sure that we will ALL be able to go. So, that has brought up many opportunities to discuss what they would like to do again, is there anything they would not want to do again, what color magicband would they get if they went again, etc. So they have been helping with the planning, just unknowingly. My DH has made the comment he would love to just not tell them at all, until we loaded up to go. I just don't see that as feasible. We've never been a big countdown kind of family, but they both really enjoyed the Avent calendars someone bought them last year. So, I was just thinking about if we did some kind of countdown, like a paper chain or something, but tell them we're going in the summer, and have the count down for that long. (Which will also help if they happen to talk to their cousin about their gift.) But either a couple days or a week before the trip, just say something like, "On second thought, let's just leave this weekend instead!" and take off all the excess chain!