When my kids were those ages, a *surprise* trip to
Disneyland would probably have been overwhelming to the point of making them feel really unsettled.
For my own children (who are now aged 7, 10, 14, and 20), I wouldn't have "sprung" that huge of a surprise on them until they were probably 6 or 7 years old (especially if that "surprise" trip was going to involve flying, or any other major experience..which can be overwhelming on its own for some people).
My kids have been to DLR many, many times...and our trip next weekend will be the very FIRST time we have surprised them with it. And the only reason we're doing that is because we have a good cover story...my nephew finishes his deployment to Afghanistan next week, and we are driving down so the kids can see him when he gets home. His parents happen to live half an hour from DLR, so we can hide it at least until we get to the hotel.
For my kids and me, a HUGE part of the fun of a trip to DLR is the planning and anticipation! Truth be told, I'm actually really missing getting to share that excitement and planning with my kids for this trip. I actually think it's going to take away from the overall fun a bit, to be honest.