In this paper, our collaborative project team shares design principles and lessons learned from research for designing an app to support families' joint engagement with media and promote powerful shared learning experiences. We provide a rationale, based on research literature, for why a second-screen app in particular addresses our project goals. In addition, we describe the Splash and Bubbles for Parents app components as well as the co-design process and design-based research studies conducted to inform its design and development. Finally, our team offers design principles grounded in findings from our research that may be useful to app developers and researchers interested in continuing and expanding on this work. [This report was written by Digital Promise, The Jim Henson Company, Concord Evaluation Group, and Curious Media.]