Here's my fears on surprising kids...you know your kids better then anyone. Do they LIKE surprises? Did you tell them they are going someplace else instead and are psyched about the other trip and will be very disappointed when they are told no. you are not going to grandma's! (Who they love to visit and miss...) We lied to you!!
Will they be upset and unhappy?
a long time ago in a galaxy far, far away...I was just turning 21 and my boyfriend told me we were going to Hershey park for my birthday. I slept over his parent's house (usually forbidden!) and the morning of the trip, they never woke me up early in the day to leave. I woke up at 10, and they told me.. "Oh we decided not to go...you looked so peaceful sleeping, we did not want to wake you." I was angry and annoyed...my boyfriend tried to placate me by taking me to the mall on a shopping spree... (see, my boyfriend at the time had a habit of making plans with me and then cancelling saying he did not feel like driving or like going out, typically after I had spent money or traveled to see him and it angered me on a regular basis...)
After about 4 hours moping around a really amazing mall....too unhappy to shop, I RACED home, crying and upset...to my parent's house...
...for my surprise 21st birthday party. Surprise!
You are not going to Hershey park, that was a lie we told you to get you out of the house so we could decorate and invite all friends and family to the house.
The party was awesome, I got to see all my friends and extended family and I got lots of great gifts but the surprise portion of it was not what I call ideal. ...And we never went to Hershey Park.
The point I'm trying to make is...know your audience.