Existing devices to assist upper extremity (UE) movement in infants with or at risk for motor impairments remain limited and are mainly passive devices. The aim of this project was to develop and assess the validity and reliability of the first-actuated wearable device for this population. A wearable device consisting of four pneumatic actuators (two per arm) was developed and tested on a custom-built physical model with articulated joints (four degrees-of-freedom (DOFs) per arm) based on an average 12-month-old infant's upper body. The device actively controls 2DOFs per arm (one at the elbow and one at the shoulder) and does not prohibit motion about the remaining non-actuated DOFs. Three distinct device actuator synergies, that resemble muscle recruitment strategies, were evaluated in a vertical reaching task using one arm and both arms. The device was assessed for its performance, wearability, and safety. Performance was assessed via the average duration, smoothness, and repeatability of reaching movements, and maximum range of motion per actuated joint. Wearability was assessed via kinematic compatibility to infant reaching trajectories. Safety was assessed via actuator durability. Results demonstrate the efficacy of the device and reveal key insights for further improvements.