This doctoral thesis problematizes and develops a theoretical explanation of children's experiences of participation in voluntary sports organizations, a key arena in the Nordic welfare context where social development, democratic values, and civic engagement are fostered. Although children constitute the largest group within organized sports, research indicates that their influence is limited and that adult perspectives often dominate discussions of participation. Anchored in the UN Convention on the Rights of the Child and informed by childhood sociology, the thesis positions children as epistemic agents with unique insight into their everyday sports experiences. The thesis employs a mixed methods design. Qualitatively, it explores children's perspectives on participation, showing that participation is shaped relationally, emerging from interactions among children and their trainers, as well as among peers. A quantitative approach further explores these results by developinga psychometric instrument—the Children's Experience of Relational Participation Scale—to capture variations in children's experiences of participation. By drawing on concepts of ageism, agency, and institutional logic, the dissertation shows that children's incentives and opportunities for participation vary with age. The thesis makes the following contributions: conceptually, it expands participation models to include children's perspectives; methodologically, it offers a validated tool for measuring relational participation; and, lastly, it provides an understanding of why participation is easily accomplished in some sport clubs but difficult in others.