Physical therapy sessions are usually long, repetitive, and painful. As a way to increase adherence to this kind of treatment, video games have emerged as a valuable asset due to their ability to foster engaging and flow experiences. However, the design of assistive video games requires integrating multiple layers of art and practice, ranging from game design to specialized domain-specific knowledge. In this paper, we report the design of a prototype video game aiming to train and assess the articular range of motion in physical therapy sessions, i.e., the extent or limit to which a part of the body can be moved around a joint or a fixed point. Such a video game explicitly uses Nintendo Switch JoyCons, state-of-the-art, and readily available off-the-shelf motion-based video game controls. As a case study, we sought to provide alternative ways to exercise the range of motion in burned children in a way that they could find engaging, yielding results that could be comparable to those obtained by more traditional means, such as with a goniometer. We conducted a proof-of-concept study involving a sample of domain experts working with patients from this population (e.g., physiatrists and therapists) as well as with a sample of potential end-users to gauge the perceived usefulness and adoption potential of the conceived solution. Our results are highly encouraging, providing more evidence of the effectiveness of (serious) video games for rehabilitation.