Constraint Therapy for Brachial Plexus Birth Injury

The goal of this clinical trail is to learn if constraint physical therapy is a better treatment than normal physical therapy for infants with a brachial plexus birth injury. The main question it aims to answer is: If constraint physical therapy can give participants earlier and better function than normal physical therapy. Researchers will compare participants receiving constraint physical therapy to participants receiving normal therapy to see if their strength and recovery is better. Participants/families will: * Undergo constraint or normal physical therapy for 5 weeks * Wear a research motion tracking device to measure movement * Answer questionnaires about if they see improvement in arm use * Visit clinical 1 and 6 months after therapy for checkups and tests * Obtain medical imaging of their shoulders, brain and spinal cord