Background: A key feature of engineering design is collaborative, deliberate decision making that takes into account information about design options. K-12 students need opportunities for this kind of decision making if they are to meet the learning standards for engineering set out in the Next Generation Science Standards. Purpose: This qualitative study sought to propose and operationalize a definition of reflective decision-making among elementary students. We investigated how urban elementary students enact reflective decision-making in a formal engineering design curriculum. Method: We used naturalistic inquiry methodology and video recorded seven Engineering is Elementary design challenges in four classrooms. Students worked in small teams, and we focused on their planning and redesign phases. Maximum variation sampling, constant comparative analysis, and microethnographic accounts demonstrated the diversity of resources students utilized in their decision making. Results: In student discourse, we found evidence for six reflective decision-making elements: articulating multiple solutions, evaluating pros and cons, intentionally selecting a solution, retelling the performance of a solution, analyzing a solution according to evidence, and purposefully choosing improvements. The discourse patterns used to enact these elements both supported and interfered with students' achievement of design goals. Conclusions: Our results suggest that during engineering design tasks, young learners working in small teams can respond productively to opportunities to engage in sophisticated discourse. However, further work is needed on tools and strategies that support reflective decision-making by all students during engineering design in elementary school.