One in 36 children in the United States has autism. Numerous robotic intervention systems have been proposed for children with autism. It is widely acknowledged that engagement with the robotic system is important for intervention success. Visual attention toward the robot can be used as a proxy for engagement. However, less is known about how to maintain and enhance visual attention within a robotic system, and how the variation of visual attention may influence adaptation of dynamic intervention protocols to improve outcomes. Therefore, in this work, we propose a new metric, System Capture Ratio (SCR), that can be automatically quantified in real-time, to measure a participant’s visual attention within a robotic system designed for joint attention intervention for children with autism. Then, we demonstrate that compared to a static intervention protocol, a dynamic intervention protocol can help sustain visual attention and thus achieve a significant performance improvement. The results support the implementation of adaptive and dynamic robotic intervention protocols for autistic children that are based on SCR, offering suggestions on designing effective multi-session studies for autism intervention.