A 14-month-old boy is brought to the orthopedic clinic with a 3-month history of limp and left groin pain. His mother reports he was born breech and was not screened for hip dysplasia in infancy. On examination, the left hip shows 20° of abduction (compared to 60° on the right), positive Trendelenburg sign, and internal rotation is limited to 10°. Plain radiographs show a shallow acetabulum (acetabular index 35°), lateral displacement of the femoral head, and delayed ossification of the femoral head epiphysis. What is the most appropriate definitive management?