The sociopolitical turn and increasing interest in the history of mathematics education have contextualized mathematics education practices in broader political projects and histories to problematize mathematics education research itself. Despite this problematization, and despite the centrality of the idea that research should improve practice to our field, the relationship between researchers and school teachers has not received much critical examination. To explore this issue, we investigate two progressive reforms advocating increased student agency against “traditional” or “rote” instruction. The Perry Movement and the New Math both occurred amid political shifts which influenced how academics envisioned “the child” as the target of curricular interventions and teacher education intended to increase students’ mathematical agency. We argue that through reforms, academics constituted and reconstituted the “child” as an abstract object of investigation through the privileging of scientific methods, curricular reforms, and conceptualization over teachers’ daily work at improving instruction. In this way, progressive reform movements displaced ownership of knowledge about children’s mathematical learning from teachers to researchers, invisibilizing teachers’ labor and expertise. We point to enduring consequences of the reallocation of knowledge about teaching from teachers to academics.