I did this for a work trip. You might try this: (though you have older ones. Which means you would need someone else to bring the suitcases to the airport)
I had to go to Washington DC during the kids April vacation. My DW and I decided that we would all go, and take the kids around DC. However, we did not tell them. Instead of flying, I take the train. We packed the kids, and told them we were going to the train to see me off. We took all of the bags across to the other platform with very little questions. When we got on the train, my DS gave me a hug, and said dad we need to get off. My DW and I said surprise, we are all going to Washington DC.
Not sure if that would work for you with the older ones. I know that either my DW or I would invariably pack the wrong clothes for the older ones, but might be worth a try.