Recent developments in multilingualism research urge us to move beyond seeing bilingualism as 'double monolingualism' and towards "translanguaging," which conceptualises language as a bundle of socially constructed linguistic resources that individuals can deploy to make sense of their multilingual world. Despite this theoretical development, the monolingualist ideology remains strong in child language acquisition and bilingual education. The One Parent One Language policy (OPOL) which I examine in detail in this paper is one of the most commonly practiced family language policies; it requires parents to constantly monitor their language practices at home based on the assumption that language is a fixed, countable, and complete system. Part of a broader ethnographic research with Japanese-English multilingual families in the UK, this article focuses on a single-family case study, through which it aims to critically examine the way in which OPOL is negotiated and exercised in situated language practices in the family home. Focusing particularly on language interactions between two pre- and early-school age children and their mother, the paper shows how the strategic and creative employment of linguistic resources by children undermines monolingualist dogmas that OPOL is reliant on. The paper thus makes a significant contribution to the empirical study of "translanguaging."