<p>Most educational research on argumentation comes from science, with argumentation in mathematics tending to focus on proof. We contend that argumentation can be used productively in learning mathematics even at the primary level. A research study was designed to explore children's development of argumentation in an Australian primary mathematics classroom. The classroom of 23 children (aged 9-10) had regularly used an inquiry-based approach to address extended, complex, ill-structured problems. The children's discussions and use of evidence is reported as they considered contentious media claims. The results of the design-based research study suggest that the children became proficient with Toulmin's argument framework (simplified). They were able to use this framework to plan, implement and defend the outcomes of a mathematical investigation they designed to provide evidence for or against the media claims. The paper highlights benefits and challenges with which student grappled while making and substantiating their final claims.</p>