Hi Marina:
Mid-December is a wonderful time to visit Walt Disney World. Schools are generally in session, so the crowds tend not to be as extreme during this period. Also, the Parks and Resorts are decorated for the Holidays. The weather in Orlando in mid-December usually hovers around 70 degrees fahrenheit or maybe a bit higher during the day. However, during the evening and certain cold snaps, the weather can really dip low, to the 40's or even lower, so remember to bring layers of clothing. Tee shirts and shorts are likely fine in the mid-day, but long sleeves and long pants may be needed at night. While the weather can get crisp in mid-December, it's usually not too cold, and if the weather gets too chilly for your liking, a jacket and a cup of hot cocoa should warm you up. And remember, Disney resort and waterpark pools are heated all year long, so unless it's very cold, you should be able to take in a swim, especially since you are from the North and can likely handle a little chill better than some of the locals. Precipitation is relatively minimal in December as well, but since it's Florida, it can always rain, seemingly at any time.
Your best bet to catch any of the Shake It Up theme is at
Disney Channel Rocks concert at Hollywood Studios, which features tunes from Shake It Up, Hannah Montana, Camp Rock 2: The Final Jam, Sonny With A Chance, JONAS, StarStruck and more. Pick up a Times Guide at Disney's Hollywood Studios to confirm schedule and show times.
Enjoy your vacation.