In France, dominant monolingual discourses and teachers' lack of knowledge about bilingualism and second language acquisition often result in 'French only' policies in classrooms including in pre-school classrooms where some emergent bilingual children speak a language other than the language of schooling and very little or no French. These policies can be detrimental to emergent bilingual children's language development, identity construction and overall long-term academic success. However, teachers in multilingual classrooms also have the power to exercise their agency to support children's bilingual language development and to implement practices which empower them and their families. This paper focuses on the ways in which one pre-school teacher working with three and four year old emergent bilingual children asserted her agency in the classroom despite institutional constraints. It investigates the beliefs, experiences, aspirations and knowledge which underpinned her sense of agency, the ways in which her actions empowered the children and parents and discusses the implications of the study for initial teacher education and continuous professional development.