Hi Sarah! The Harmony Barber Shop in the Magic Kingdom just recently began accepting appointments, but they're not required. You can make an appointment by calling 407-WDW-DINE or 407-WDW-PLAY; a credit card guarantee is required and there is a 24-hour cancellation policy. (If you fail to cancel and don't show up for your appointment, your card will be charged $10.) Walk-ins are still accepted as well, but be prepared to wait, as the Harmony Barber Shop is quite popular!
The barbers there are licensed cosmetologists -- girls, as well as boys, men, and women, can get their hair cut and styled there just like at home. In addition to haircuts, girls, boys, and adults can have their hair styled with colored gel and sparkly pixie dust! My own daughter had her first haircut there, and also has had her hair styled into cute braids sprinkled with colorful pixie dust and Mickeys.
Hope this helps! Come visit us again at the Moms Panel if you have more questions!