Many young children of Mexican heritage enter U.S. schools with knowledge of two language systems and emergent biliterate abilities. Bilingualism in young children may go unnoticed when instructional practices favor English. This case study describes how Lucía's bilingualism and emergent biliteracy parallel competencies essential to literacy development. The language samples highlight the influence of adult-mediation interactions in everyday conversations as well as during playful dialogues, characterized by the incorporation of fantasy and symbolic play. The findings show how adult-mediated dialogues challenged the young bilingual to engage her metacognitive and metalinguistic awareness when positioned to meet communicative goals and to solve conceptual conflicts, both verbal and written. These opportunities guided the young child to also critically think about language and to expand her notions about literacy. We draw implications about how educators can leverage these metacognitive and metalinguistic competencies as educational resources.