I work at a Gym Day Care and have many stories of mother's going through the same thing. One mother would bring her daughter in (1 year old, I think) and cheerfully and smiling say "bye bye, mommy is going to work out and would be back". Then she would leave for 5 minutes (daughter would cry hard the entire 5 minutes) and then come back smiling and cheerfully to pick her up and take her home.
Next day she did it for 10 minutes, day after 20 minutes -etc etc.
By the 4th or 5th day, the little girl was fine.
I agree with Calum's mom that you should NOT sneak out. That will break her trust and when she looks around and you are gone....IT CAN GET UGLY!!
It also helps if the staff engages your dd and distracts her after you leave. Many times children/babies come in crying and within 5 minutes they have found something or someone to play with and actually look disappointed when the parent's come back.
Good Luck. Feel free to PM me with any other questions.