As a way to examine the validity of deficit perspectives on multicultural children in South Korea--namely, children of mixed parentage who are said to suffer from speech problems and school failure, this study examined their everyday and academic Korean proficiency and its association with their educational achievements. The primary data sources for this study included fieldnotes produced over 11 months of fieldwork at three focal students' homes and schools, their academic records and their academic vocabulary test results. Adopting an ethnography of an embedded case studies method, this paper employed inductive thematic analysis. This study revealed that multicultural children did not exhibit any difficulty in communicating with others in everyday Korean but that they had varying degrees of academic vocabulary mastery. This study further confirmed the tight relationship between academic vocabulary knowledge and school success. In addition to the discussion of conditions that could widen the gaps in academic vocabulary repertoire among children, this paper called for more explicit academic vocabulary instruction for children in need, regardless of their multicultural background.