The present paper documents the design and development of a mobile mathematics application targeted to improve magnitude representation skills. Educational experts worked together with an app developer with the goal of creating an educational app as a math learning tool for children 5-8 years old. The description of the app design processes includes five core elements that we believe are central to the creation of a theory driven educational app. Creating a theory driven educational app is a difficult task; it involves a set of complex decisions as illustrated in this article.