Ankle fractures can be challenging injuries in a child because there may still be an open physis (growth plate) that could be disrupted. The result can be deformity, a leg length difference, impingement, and overload of one side of the ankle.
When planning the most appropriate treatment, pediatric orthopedic surgeons often use a classification system known as the Salter-Harris Classification method. It is used to determine type of fracture, amount of displacement, amount of growth left, and the best way to manage the problem.
When using the Salter-Harris classification system, there are five major types of ankle fractures (I through V). Each number signifies the severity of the injury and the amount of growth plate involvement. The classification numbers also give an idea of the risk of growth arrest (e.g., low risk with Type I fractures, high risk with Type V). Fortunately, Type V fractures are rare.
The two main goals of treatment are to maintain optimum function and limit risk of physeal (growth plate) damage. Besides damage to the growth plate, trauma to the surrounding soft tissues must be assessed as well. Ligamentous damage can create an unstable ankle. But usually, the ligaments are stronger than the weak, growing physis. So in the growing child, physeal injury is more common than ligamentous damage.
The surgeon will look for any damage to the blood vessels, nerves, tendons, and muscles, and also rule out the presence of other bone fractures in the foot. A large amount of swelling may mean a delay in surgical correction or cast immobilization. The risk of infection and difficulties with wound healing are too great to intervene with early surgery. This may be the case for the child you are asking about. If you are the parent of guardian of this child, you can certainly ask this question of the attending physician(s). There is likely a very reasonable explanation for the planned delay.