My kids knew in advance about the first trip - since they had no idea what to expect (and it's hard to prep them on what to expect), a surprise on that trip wouldn't have been much fun.
The second trip was a spur of the minute trip - we had two other vacations fall apart on us (flooding in the north, when we were going to Hershey, PA and then a hurricane near Destin) so we decided to swing a Disney trip. Since we wouldn't have to keep it a secret for long, we decided it would be fun. Both DS's were immersed in their video games while we passed all the billboards for Food and Wine and
MNSSHP, so that was good. I have a great video of us getting to the big sign over the road and DS#1 going nuts when he saw it - DS#2 complained that he wanted to go to a hotel! Lol! Not the reaction we were looking for, but hilarious nonetheless. Once he realized the hotel was in WDW he was okay.
We had a double surprise for them that year - the grandparents showed up for the second half of the trip, so they absolutely loved that.
Last year's trip was planned far in advance and we kept them in on the planning and choosing ADR's, etc. My parents were also going to join us for this trip, so they knew about that far in advance - partly because we didn't think all of us could keep a secret, but partly because my mom would be in a wheelchair in the parks and we wanted them to prepare for seeing grandma like that (she walks with a cane for short distances, so it wasn't even an issue for them).
This year we're driving across the country to California and they know about that trip, but they have no idea
Disneyland even exists, so the four-day stop there should be an amazing surprise for them!