Health literacy enables people to access, understand, appraise, remember and use information about health. Critical health literacy is a domain of health literacy and enables individuals and communities to engage in social and political processes for action on the determinants of health. Promoting critical health literacy early in the life course may contribute to improved health outcomes in the long term. Yet children's opportunities to develop critical health literacy are limited and tend to be school based. There are recognised barriers to implementing critical health literacy interventions in schools. The aim of this study is to broaden the range of settings based approaches available by exploring the potential role of public libraries as community-based supportive environments for children's critical health literacy. The study was designed in consultation with a Children's Advisory Group of eight children. The setting is a public library system in England. The theoretical and methodological framework is institutional ethnography. Semi-structured interviews were conducted with 13 children, and semi-structured text-elicitation interviews were conducted with 19 public library staff and community stakeholders. The data were analysed through the lens of a conceptual model based on the literature. The model provided an organising framework for the data and informed a keywords approach to analysis. The findings show that texts produced by the public library sector refer to health literacy support for local communities as part of the public library service offer, but the library was not perceived as a setting for health, and schools influence the health literacy development opportunities available for children there. Critical health literacy was seen as beyond the remit of the library, although one activity promoting critical health literacy was identified. This activity acknowledged the wider determinants of health, was accessible to children, involved children in how it was run, and facilitated children's informed action for health. A revised conceptual model is proposed that identifies the necessary conditions, or antecedents, for public libraries to be a supportive environment for children's critical health literacy development. The revised model situates the public library in a coordinated, multisetting (supersetting) approach with other settings where children spend time, including but not limited to schools. The study advances the theory and application of a supersetting approach to the development of critical health literacy in children and highlights the possibilities of non-traditional settings for health. It also contributes to the ongoing development of institutional ethnography and health literacy research with children.