This paper examines how children exercise their agency vis-à-vis their mother's beliefs and practices of bilingualism, especially code-switching, at home and elsewhere. The data were mainly collected through audio-recordings of family conversations both with and without the presence of the researcher and semi-structured interviews conducted during 19 visits to two families with Japanese-speaking mothers and 12-year-old English-Japanese bilingual children. An analysis of the data reveals that, despite there being a difference in language practices and backgrounds, both children discursively negotiate and construct their bilingual positions at home through their flexible use of the two languages. Furthermore, their language practices are not a passive response to their mothers' beliefs, but an exercise of their agency through contestation, negotiation and redefinition of the mothers' beliefs. In particular, this study has identified two types of agency: (1) construction of the positive self-image and negotiation of the parent-child relationship through the active use of English; and (2) resistance to the mother's monolingual policy through a flexible use of the two languages. The data also provide insight into the situated and multidimensional nature of agency by showing changes in the children's practices depending on the context, such as in conversations with the researcher.