We surprised our kids two years ago.
DS was 4 and DD was 8. We told them we were flying to their grandparents.
DD started putting two and two together at the airport when they were annoucing Orlando and saw other kids with Mickey ears getting on the plane. But we kept up our story.
At MCO they were sound asleep and we just all jumped into the taxi to get to BCV.
At BCV DD woke up and felt she was dreaming. DS was totally confused.
In hindsight I wish I had not done it. While we got a big kick out of it, we missed the excitement of planning the trip with them. DS was too young to be concerned, but our 8-year-old felt disoriented the first day, walking around saying "I can't believe I'm here..." And finally, they love going to see grandma, so they were almost a bit disapointed we didn't go there, (but just a bit.)
I like the surprise idea, but I'd break it to them a few days before. Heck they need the chance to get as excited as we are, don't they?