We wanted to surprise our kids as well and originally we were going to wait until the day of but we chose to tell them before hand.
We took our kids to the play "Peter Pan". They were SO SO excited to see the play. I had called the actor playing Peter Pan one week prior and asked her if she would assist me in surprising the kids.
After the play was over, we were on our way out and the kids grandparents led them to the back door where the actor would come out.
I had created a letter with
Disneyland logos which was addressed to the kids and the return address said it was from M. Mouse, the return address was toontown, anaheim California.
Peter Pan came out and sprinkled pixie dust on their heads and asked them to hold some dust as well. She read them the letter which said things like, "Minnie and me can't wait to see you." The kids thought we were just going to Vancouver (we live in Canada) and the letter said, "you think you are going to Vancouver but the truth is your parents are going to surprise you and take you to Disneyland. You get to take your very first plane ride". We saw the play exactly one month prior to our departure date so that gave the kids one month worth of countdowns.
They were thrilled. My son looked like he was in shock and my daughter was so thrilled.
A crowd gathered from the theatre and people were crying and the crowd all clapped and cheered.
I had contacted the Vacation Planning Department and arranged to have them send postcards to the kids and within a few days of finding out about the trip, each of my kids received a post card saying "We are looking forward to your visit, love your pals at Disneyland".
It was a very magical night.
I am sure none of us will every forget it.
Good luck with your trip.