My parents were always straight with me. They never tried to hide or sugar-coat anything. I was always told when I had to go to the doctor's or the hosptial. I was always allowed to express my feelings about it and they did their best to make me feel better.
The older I get the gladder I am they did things this way. I don't think tricking children into going to the doctor, or surprising them with it is very good for them. Plus, after each appointment or hospital stay I always got a treat, be it a meal out or a new book. That always made things go better!