Involving children in participatory health research (PHR) provides exciting opportunities to gain insights into their perspectives and capacities and encourages them to make a meaningful contribution to issues affecting their lives. It is underpinned by a rights-based approach, where children’s evolving expertise is valued. In PHR, children are not just research participants – they are co-researchers. This raises challenges and ethical issues on several levels, including the nature of child engagement, the role of adults, and methods to promote child agency, while also ensuring their safety is not compromised. This chapter uses real international exemplars to illuminate the complexity and debate on theory and practice in relation to PHR with children. These examples reflect the flexibility inherent in the approach to accommodate culture and context. They also highlight the need to avoid tokenism, which might undermine children’s agency, and encourage first attempts, reflexive practice, and progressive improvements in PHR competence. Children’s right to subvert or deny the participatory process is recognized as a challenge to the adults involved but is also a way to realize impacts and outcomes that are more relevant for those children. Finally, Kids in Action is presented as an international network of PHR projects involving children to promote best practice, develop and share resources, link similar projects to maximize impacts, and provide a platform for children’s voices in relation to global issues.