Language choice and code-switching are commonplaces for peer interaction among bilingual children from various language backgrounds in Hong Kong. However, few studies concern these linguistic practices of immigrant children proficient in Mandarin and English. This study adopted a qualitative research approach to investigate three Mandarin-English preschoolers' language choice and code-switching by analyzing their conversations in interaction with a Cantonese-English bilingual peer under the framework of translanguaging. The findings reveal that these Mandarin-English preschoolers' priority language for interaction is English, indicating that choosing a shared language is an essential communicative strategy for integrating with their Cantonese-English peers. The factors for those language choices include social environment, interlocutor's ability, and sense of identity and belonging. Both inter-sentential and intra-sentential code-switching are adopted in verbal interaction. However, intra-sentential code-switching occurs more frequently and with greater diversity in Mandarin-based sentences, where nouns are the most easily switched component, followed by verbs and adjectives. This study enriches the study of bilingual children in Hong Kong, extends the application of translanguaging to peer interaction outside the classroom in multilingual contexts, and provides pedagogic implications.