This study examines the role of agency in a young trilingual child's language choice in interaction with her Mandarin-speaking grandparents. The child was born in Japan to a Chinese mother and a Japanese father. English is used as a lingua franca in the family. The study demonstrates how the child asserts her agency to negotiate the decisions and efforts made by the grandparents. The bond of the heritage language and culture, and the value of child trilingualism are strongly desired and explicitly implemented in the grandparents' monolingual or 'bilingual-like' discourse strategies in dealing with the child's mixed codes. Meanwhile, the child's flexible language use is not a passive response to the grandparents' strategies but an exercise of her four significant senses and behaviour, which are: (1) resisting through no response; (2) moving on in a dual-lingual conversation; (3) assisting the grandparents to decode her non-Mandarin speech; and (4) modifying the language choices of herself and others. This study suggests that language choices of trilingual children are complex. It also provides empirical evidence that grandparents provide an important incentive in the planning of family language policy.