Aw, that sucks. The best part is in the surprise.
Were they excited at least?
The worst ruined Christmas surprise in our house was one year my parents were surprising us with a trip to Disney World (the one and only trip we EVER took as a family). Well, didn't the travel agent call one day, I answered the phone, and she told me to tell my mom that the plane tickets to Florida were ready for pick up!
Even though we knew what our present was going to be, we were still very excited. If it's something they really want, they won't care. Put it this way...at least now they don't have to worry that they won't get it!