Young children with motor disabilities face extra obstacles to engaging in movement and initiating social interaction. A body-weight support harness system (BWSS) allows a child to take steps, explore the environment, and interact with people and objects, but further research is needed to understand how this type of system can help children with motor disabilities. Assistive robots have the potential to keep a child engaged and motivated during physical therapy sessions with a BWSS. We conducted a case study over three and a half months to understand if the BWSS alone and if the BWSS with an assistive robot could promote child movement and engagement. Our results show that the child tended to increase their amount of movement over each session with the BWSS. The assistive robots used in this study also tended to keep the child engaged. The products of this work can benefit clinicians and researchers interested in early mobility intervention technologies, as well as roboticists working in the child mobility domain.