In this paper, we present a research approach that makes visible how young children in Chinese immigrant families muster their multilingual, multimodal and multisemiotic repertoires as they interact with distant family and friends on social media. The approach brings multimodal social semiotics into conversation with translanguaging to problematize the notion of languages as bounded systems, and to illustrate how emergent multilingual learners deploy their knowledge of the features of different language scripts and modalities to maximise their communicative capacity. We focus on the emergent translanguaging practices of Chinese immigrant children when using WeChat--a popular Chinese social media that is widely used by young families in their everyday language and literacy practices. Reporting on a study of nine immigrant families in southeast London, we home in on one boy aged eight years and his younger brother aged six years, with mixed Chinese (mother) and Portuguese (father) immigrant heritage. Through fine-grained multimodal analysis of online exchanges between the older brother and contacts in their mother's WeChat network, we illustrate the multimodal, translinguistic and polyadic nature of his language use in practice and reflect briefly on the disjuncture between his home uses of multiple languages and his schooling. We also consider how the younger brother is socialised into translanguaging practices by observing and occasionally participating in his older sibling's online chat. The findings address a gap in research knowledge by illustrating how social media can enrich opportunities for young children's emergent translanguaging practices and heritage language learning.