The aim of this study was to assess whether and how a sustained instructional focus on argumentation might improve children's understanding and application of key epistemic criteria for scientific arguments. These criteria include the articulation of clear, coherent causal claims, and the explicit justification of such claims with appropriate evidence. We show a mixed-age class of 8-10-year-old children improved in their ability to both construct and evaluate arguments, especially in the ways they met evidentiary criteria. We locate these improvements in their classroom's development of a number of norms for "good arguments" that focused on evidentiary standards. We summarize how students' appropriation of specific norms around showing evidence and justifying evidentiary relations produced these outcomes. We frame these findings in terms of their implications for promoting argumentation in classrooms, children's capacities for engaging in such argumentation, and in relation to the development of informed views about the nature of professional science. (Contains 5 tables and 6 figures.)