Background: Guidelines for the successful implementation of two-way dual language bilingual education (DLBE) programs foreground the role of program leadership, hiring and preparing culturally and linguistically competent teachers, family involvement, and the alignment between curriculum, assessments, and instruction in both languages. Absent in this framing for program implementation and success are the voices, lived experiences, and everyday contributions of immigrant children who take part in such programs. Focus: Grounded in a critical orientation to immigrant childhoods, this article explores how a group of 70 Brazilian immigrant children (ages 5-7) navigated a new two-way DLBE program (Portuguese-English) in the U.S. state of Massachusetts. By looking into how the children interacted and participated in class, we foreground how they contributed to the maintenance of the program as two-way DLBE implementers. Research Design: This article draws from a larger ethnographic research study conducted over three school calendar years (2018-2021). It leverages field notes from weekly classroom visits in the two-way DLBE program and semistructured interviews with 18 school staff members. Data sources were coded using thematic analysis, with attention to how immigrant children's actions contributed to teachers' instruction and how these contributions were perceived by educators and other staff members. Findings/Conclusions: Brazilian immigrant students in K-2 classrooms contributed daily to the implementation of their two-way DLBE program through language brokering across Portuguese- and English-medium classrooms, facilitating peer participation in class, and invoking transborder memories in ways that complexified the ongoing classroom discourse. However, two-way DLBE educators and other staff members characterized newcomer students from Brazil as bringing key linguistic assets to school while positioning the children born in Brazilian immigrant households in the United States as "caught in-between" languages and undermining the program implementation. These findings suggest the need for a holistic focus on immigrant childhoods in two-way DLBE programming.