When are you planning to surprise them? Are you going to tell them a few weeks before, or just let them figure it out when you arrive? I'm just thinking that, though surprises are good, sometimes the planning and preparing for Disney is fun too.
One year (when I was about 5 and my sister around 8) our parents surprised us with a trip to Disney. We were doing a drive down holiday, going along the I95, visiting any museums that took our fancy, but they didn't tell us we were going to Disney (so they could avoid the week of "are we nearly there yet"s

). When we got there I was just pleased to be in Disney, but my sister found it hard to deal with the sudden mental adjustment (she's NT). It took her a couple of days before she really got into the swing of the Disney holiday, because she hadn't had time to prepare herself. Just thought I'd throw that experience in, as it might help.
As for planning without telling them, that's a tricky one. If he were a bit older, I might suggest introducing him to Virtual Magic Kingdom (an online, multi-player computer game run by Disney), but he's probably too young for it. You might be able to try the "I know how much you love the planning video, so I thought I'd show you these" line, but the girls might get suspicious. I'll have to think about this one...
