We did this about 5 years ago, and it was the greatest surprise I've ever pulled off as a parent. Our two youngest were 12 and 8 at the time.
DH and I had the trip planned for months - just the two of us. A few weeks before leaving, he found out he had won a trip to the Virgin Islands through work for later in the fall, and then we started feeling REALLY guilty.
So, we decided to buy two more airline tickets and take the kids with us. They were under the assumption that they were staying with their older sister for the week, so they were packing right along with us. It was easy to trick them!! I ended up packing a backpack for them with goodies for the flight, along with another suitcase of extra clothes. I bought them some new clothes, but they just figured that they were for the upcoming school year. TOTALLY CLUELESS!!!
Our oldest daughter called them the day before and finished setting them up for the fall. She told them that her and her husband would take them to Six Flags Great America for the day, so they would have to get up really early and ride along to drop mom and dad at the airport, and then they'd be on their way to Chicago. Of course - they fell for it!
We arrived at the airport, they jumped out to kiss us goodbye and fight over who was going to ride in the front seat. That's when I knew we had them!
I told them to give their sister a kiss and tell her goodbye. They looked at us kind of confused.

I then told them that they needed to kiss her goodbye, because they were going with us. The looks on those two faces is something I'll never forget - shock and then some major excitement. It was the BEST day!!!!!
