This paper presents the design of an mHealth application for prevention and early intervention of childhood anxiety. The application is based on REACH, a preventative-early intervention protocol for childhood anxiety. This paper describes the multidisciplinary design process, sharing lessons learned in developing an effective mHealth application. This mHealth application is unique due to participant age, preventive-early intervention focus, and utilization of mobile technology in a situated manner. A design process inspired by user-centered leveraging key informant interviews was used to identify application features, including game based strategies and an animated motivational avatar. Validation was performed through external review and a usability study performed with target end users of the application. Results suggest overall satisfaction, ease of use, and increased motivation.