DH and I took dd (4) for the first time Nov 05. We didn't have a plan in mind of how we were going to tell her, we just kept waiting to see if she could figure it out. On the plane, she knew she was going on vacation, and many other passengers asked her if she was going to WDW. She still didn't seem to catch on, it wasn't until we were at the gates of MK during the opening ceremony that she screamed, I'm at WDW!
For me, it was easier to plan everything on the sly (well, not so much, dd's obviously kind of clueless, lol) than to deal with "Are we going yet?" everyday until vacation time. But you will get some great ideas from this board about how to surprise them, whenever you do it.