Drawing on Cultural Historical Activity Theory (CHAT), funds of knowledge, and third space, this article presents a model for practitioners and researchers to think about how Latina/o, bilingual children develop explicit mathematics strategies through multilingual and multigenerational interactions. Using data collected through fieldwork in an urban afterschool club, this article illustrates how nonschool activities that draw on children's funds of knowledge (especially Spanish use) can foster robust problem-solving interactions. When activities are purposefully designed to build on productive tensions between the sometimes conflicting goals of participants, these "third spaces" lead to movement in the zone of proximal development. Multiple vignettes from the Counters game are presented to demonstrate how parents, undergraduates, and graduate students purposefully interact with children to facilitate movement from "informal" spaces to more "formal" spaces of mathematics. With the population of Latina/o bilingual children rapidly rising in urban schools throughout the United States, it is essential for practitioners and policy makers to adopt additive rather than subtractive views about the knowledge, linguistic, and cultural tool kits of Latina/o bilingual learners. (Contains 1 table, 3 footnotes and 6 figures.)