Learning an instrument can be an exciting milestone in a young child’s life, yet the considerable time and effort it demands often lead many students to quit. This study examines the factors that sustain young children’s engagement with a string instrument once the initial novelty has subsided. Guided by Deci and Ryan’s (1985) self-determination theory and Csikszentmihalyi’s (1990) flow theory, this qualitative instrumental case study investigates the lived experiences of six third-grade musicians in an after-school orchestra program at a Jewish day school in Los Angeles, California. Recognizing children as “experts and agents in their own lives” (Clark & Moss, 2011), the study employed multimodal methods, including semi-structured interviews, a mini-lesson teaching activity, an arts-based drawing prompt, and flow observations of group rehearsals to center children’s perspectives. Through these methods, six themes emerged: (1) meaningful connection through musical engagement, (2) self-directed musicianship, (3) navigating the joys, struggles, and strategies of learning an instrument, (4) the role of family in supporting young musicians, (5) flow as a dynamic system of skill and social connection, and (6) the challenges of navigating a digital world. These themes position children as agentic and competent participants in their own musical journeys, revealing learning motivation as a lived and negotiated process rather than a fixed trait. Insights from this case study found that sustained engagement occured when autonomy, competence, relatedness, and cultural meaning converged within supportive environments, such as those found in families and schools. Together, these findings suggest that children’s motivation to persist with an instrument is best understood not as a simple intrinsic-extrinsic dichotomy but as a dynamic continuum shaped by social and cultural context. Implications are offered for music educators, program directors, and parents seeking to support children in developing sustainable, joyful, and identity-affirming relationships with their instruments.