Several families I know have surprised their preschool age/young elementary age kids the morning of the flight, or even after the flight ("we are visiting friends"), or somewhere along those lines. I don't know how they do it.
I pack for days, plan forever, and love all the prep.
My 5 year old is very inquisitive, and perceptive, and I'd have to literally lie to keep something like that a secret I think, and I wouldn't want to... but even if I was able to keep it a secret, I'm not sure I would - at least at this age.
Here is why: She begged to go on this trip as soon as she saw I looked at the website, and started planning it... It was actually very helpful for me to find out what her dreams, hopes, priorities and fears were ahead of time... We looked at hotels, pools, the parks and rides together, and that was fun! Watching her get her little sister excited about the trip each time we had looked at something was adorable as well. We even watched some rides together ahead of time on the internet and without doing that, she would have refused to go onto those rides. So that is something to consider as well. She has great memories now of having "braved" these rides!
This is a kid that LOVES surprises by the way. However, when I asked her a long time back (then four years old), if she ever wanted to be surprised with a vacation/traveling (she knew what that meant), she thought a good while and then said; no! "Because I would miss out on all the fun planning!"