This study explores the language use of Southeast Asian marriage-migrant mothers in South Korea with their mixed-heritage children, and the challenges related to heritage language (HL) transmission. Drawing on interviews with nine women, the study finds that they encountered multiple obstacles to teaching the HL to their children. Their Korean family members regarded HL learning as a hindrance to the children's success and discouraged the development of their bilingual and bicultural identities. Moreover, the mothers themselves promoted Korean at home because they believed it was necessary to the children's academic and social success. Their strong desire for their children to assimilate and conform to the dominant language and culture was influenced by a mainstream society in which mixed-heritage children are vulnerable to social exclusion. As a result, the participants' children were prevented from receiving rich HL input and lacked fluency in their HLs. This study aims to improve our understanding of the factors that facilitate or hinder HL transmission and development in the context of mixed families in South Korea.