Not necessarily. Ganglion cysts that aren't causing pain or other problems in children tend to go away with time. A recent study showed that children less than nine years old got complete resolution of ganglion cysts within a year from the time the went to the doctor. And the only treatment was to watch and wait.
Surgery usually isn't the best treatment for children with simple ganglion cysts. In children, about half of the cysts treated by puncturing with a needle (aspiration) come back. When surgically removed, about 35% of the cysts return. But many cysts simply go away with time. The high rate of recurrence, along with potential complications of surgery make observation of ganglion cysts in children the best bet.
Be sure to check with your doctor to make sure he or she knows the location and size of the cyst. Together you can monitor improvements and develop a plan if the cyst doesn't go away.