Over three decades ago, National Council of Teachers of Mathematics (NCTM, 1991) proposed that "for students to develop the ability to formulate problems, to explore, conjecture, and reason logically, to evaluate whether something makes sense, classroom discourse must be founded on mathematical evidence" (p. 34). As math interventionists supporting small groups of Grades K--4 students, the first three authors wondered how they could still provide their students with opportunities to engage in reasoning-based discourse despite the directive to have students who had yet to learn their basic facts work in small groups. All authors were committed to uplifting individual students' thinking through positioning them to think flexibly about various strategies by developing the role of a student referee, whose main responsibility is facilitating small-group conversations grounded in mathematical reasoning. Although the authors support small groups of elementary students in their understanding of number concepts, teachers can adapt their approach to other content and grade levels.