BACKGROUND: The concept of child 'agency' has become increasingly important for international child-centric organizations, non-government organizations (NGOs) and United Nations (UN) agencies, particularly those responding to the issue of child marriage. Interventions to prevent child marriage often include awareness-raising activities focused on sharing information with children on the dangers of child marriage. Such interventions are often based on, and perpetuate, a belief that increased knowledge can lead to an increase in girls' agency. In this framing, agency is presumed to result in a 'good' decision and is positioned as a natural consequence of increased knowledge. This agency is said to enable girls to resist marriages forced upon them by their parents. OBJECTIVE: This discussion paper aims to interrogate dominant conceptualizations of child agency through an exploration of child agency narratives on child marriage. METHODS: This discussion paper is based on critical analysis of existing academic and grey (NGO and UN) literature that explores children's agency in the context of child marriage prevention. RESULTS: This discussion paper suggests that academics, NGOs and UN actors use varied definitions to describe agency. While academic analysis shows that children's agency might be contested, contradictory and fraught, NGO and UN agencies tend to narrowly frame agency. CONCLUSIONS: We suggest that adopting a broader definition of children's agency in research and implementation enables a more nuanced, complex understanding of the drivers of child marriage and the interventions required to address this practice.