Existing studies on minority language shift in China have mainly examined macro-level language policies, adult language practices, or general patterns of proficiency loss, leaving insufficient attention to how children in ultra-endangered language communities experience and negotiate highly asymmetrical bilingual resources. This study investigates the bilingual development, language socialization, and identity negotiation of Gelao children in China, whose heritage language is critically endangered and no longer widely transmitted across generations. Drawing on language socialization, child agency, translanguaging, and identity-in-practice perspectives, the study employs a convergent mixed-methods ethnographic design in Gouchang Village, Guizhou Province. Data were collected through a bilingual vocabulary assessment with 8 Gelao children, questionnaire-based interviews with 12 children, and caregiver questionnaires from 26 guardians. Quantitative data were analyzed descriptively, while qualitative data were examined through thematic analysis and integrated at the interpretation stage. The findings indicate a marked asymmetry between Mandarin and Gelao. Mandarin functions as the dominant language across school and peer domains, supported by institutional expectations, communicative necessity, and repeated use. In contrast, Gelao is rarely used as a daily communicative language and appears mainly in limited family, affective, and symbolic contexts. Children's ethnic identity construction is therefore not based primarily on stable heritage-language proficiency, but on metalinguistic awareness, family relations, cultural rituals, and affectively charged associations with Gelao. These findings suggest that Gelao children should not be understood simply as passive recipients of language shift, but as emerging social actors who navigate uneven bilingual ecologies through domain-specific adaptations. The study contributes to discussions of language socialization, child agency, and dynamic bilingualism in contexts of severe language endangerment, and offers preliminary implications for ecology-building revitalization strategies, including intergenerational digital archives, translanguaging-informed pedagogy, and youth-oriented spaces for heritage language use. .