This paper explores bilingual peer interaction during classroom activity transitions —defined as unstructured temporal and spatial configurations between scheduled lessons or other activities. Drawing on audiovisual recordings from a first-grade (Grade 1) “English-only” classroom in Japan, the study employs multimodal conversation analysis (CA) to examine how children mobilize two languages (Japanese and English) along with a range of multimodal resources to engage in (i) improvised performances and (ii) imaginary play with objects. The analysis shows that these transitional moments serve as a vehicle through which children perform heteroglossia and create a “translanguaging space.” In doing so, they display their bilingual competence, challenge the “English-only” language policy, reproduce and transform the classroom’s moral order, and socialize peers—thereby constituting the bilingual peer group. Data are in Japanese and English.