Orthopedic surgeons are specialists in repairing bones, as you already know. However, depending on the types of patients they see, their expertise within orthopedics varies. The problems that children may encounter are different than that of adults. For example, children still have growing bones, so this must be taken into account when a doctor is operating on the bone.
For younger children, the equipment a surgeon may use may be a different size and approaches to the surgical area may be different. By staying within their realm of expertise, the surgeons give their patients a better chance at a successful surgical outcome.