A rich variety of videogames promoting physical activity has followed the emergence of new full-body interfaces. Known as exergames, these active videogames are often presented in the market as a ludic substitute to traditional sport. Although they present the benefit of being engaging, to date, the content and modality of interaction of these games cannot be granted as a regular mean to do exercise. This is an issue of particular relevance when they are perceived as a valid alternative to develop children's motor skills. This paper presents the design strategies and evaluation of the "Fish Game", an exergame that has been specifically designed to spur children to execute specific types of movement determined by health experts. In a controlled assessment with 150 children, we compared the diversity of movement in the Fish Game with respect to a previously designed game. Video analysis shows a richer variety of movements was executed in the Fish Game. We discuss the limitations of our current design procedure and future avenues that could be explored with health experts to enhance it.