This study examined children's learning when they played 12 digital math games to understand how design features promoted children's awareness of the affordances in the games, and how this contributed to their learning. Elementary-aged children (N=193) played three digital math games during 60-minute interviews. We collected quantitative (pre-post tests) and qualitative (videos) data to answer the research questions. Results showed learning gains for most games. The video data reveals how children's interactions with digital math games impacted their learning, how design features in the games promoted children's awareness of the affordances, and how this related to learning outcomes. Key themes for the most salient design features were: "providing information," "manipulable math objects," and "focused constraint."