The Next Generation Science Standards (NGSS) calls for the integration of the practices of science and engineering across all science disciplines beginning in the early elementary grades. Science and engineering education research has determined that engineering design is a productive means for promoting understanding of science concepts. However, design artifacts created during engineering design problem-solving have not received sufficient attention for their potential to embody children’s science understanding. The aim of this study was to examine how conceptual development of the concepts of force and motion was instantiated in design artifacts by early elementary age children engaged in engineering design. Twenty-six children, ages 7-8, from 13 states across the United States engaged in the study from their homes. Design artifacts were considered externalized mental models with evidence of conceptual development evaluated according to the type and number of perceptual dimensions present. It was determined that the artifact could have eight possible perceptual dimensions and the addition of perceptual dimensions was considered evidence of conceptual development. Results indicate that children developed mental models ranging from 2-8-dimensions, with 23 participants (88%) adding dimensions to their mental models during the engineering activity. Video-stimulated prompted recall (VSR) interviews were used to corroborate conceptual development viewed through the design artifact, with all participants able to corroborate or partially corroborate their mental model changes. VSR was instrumental in engaging participants in the metacognitive process of reflection, a known mechanism of promoting conceptual development, which is underutilized with young children. VSR assisted some children in overcoming obstacles in problem-solving. Results are specific to the cotton ball launcher and further study is needed to improve generalizability to other engineering design tasks pertaining to force and motion.