Final answer:
A pediatric orthopedic surgeon would be paramount in treating a child's broken leg, but of the options given, a social worker would be the most important to involve for holistic care and family support.
Step-by-step explanation:
The most important specialist to involve in a 6-year-old child's care during hospitalization for a broken leg resulting from a motor vehicle accident would likely be a pediatric orthopedic surgeon. This specialist is trained to address musculoskeletal issues in children and would be essential for properly setting the child's broken leg and managing their recovery. However, of the options provided, and assuming the specialist playing the primary treatment role is already involved, a social worker might be critical in addressing any potential challenges the family might face due to the accident, such as coordinating home care, rehabilitation, or accessing community resources, and offering emotional support. A social worker would also act as a liaison between the medical staff and the family to ensure all of the child's needs are met.