This scoping review examines how narrative features and game mechanics intersect to shape emotional engagement, motivation, and health outcomes in serious games designed for children and adolescents. Analyzing 18 empirical studies published between 2015 and 2025, this review identifies recurring patterns across narrative genres, plots, structures, and character archetypes, and highlights how game mechanics are systematically integrated with narrative to construct coherent, engaging, and developmentally appropriate storytelling experiences. To synthesize the findings, we introduce the Narrative–Mechanic–Outcome matrix, a design-facing tool that maps how specific narrative–mechanic configurations are associated with different engagement and behavioral outcomes. The matrix enables researchers and developers to position their design strategies within an evidence-informed context and consider potential underexplored intersections. Findings highlight the importance of aligning narrative tone and mechanic complexity with developmental stages, particularly when addressing sensitive health topics such as mental health or identity. This study contributes both a conceptual vocabulary and a NMO matrix to reference future design and evaluation of narrative-driven serious games for youth health promotion.