Another time to refer is anytime you feel uncomfortable. If you sense something isn't right, you will never be faulted for referring the patient to a specialist. So, when in doubt, go ahead and refer.
Typically, a good history and physical examination will be sufficient, with or without x-rays, for a pediatrician to determine the best recommendations for the patient.
Although x-rays are a necessity for evaluation of most orthopedic or sports injuries, it is preferable to refer the child and have the subspecialist order imaging tests. This avoids duplication of radiation exposure for the child and the unnecessary time and expense of repeated x-rays. In addition, laboratory assays typically do not help in the evaluation of a suspected overuse injury, unless you suspect a comorbid condition such as arthritis or joint infection.