This paper focuses on developing students' informal ideas of inference and argumentation. This topic is of current interest to many researchers as well as teachers of statistics. We study fifth graders' learning processes in an exploratory interdisciplinary learning environment that uses the software TinkerPlots to scaffold students' statistical reasoning. The careful design of the learning trajectory based on increasing samples heuristics coupled with the unique features of TinkerPlots were found instrumental in supporting multiple dimensions of students reasoning about informal inference: multiplicative reasoning, aggregate reasoning, acknowledging the value of large samples, and accounting for variability. These were accompanied by greater ability to verbalize, explain and argue about data-based claims. In the light of the analysis, a description of what it may mean to begin reasoning about inference by young students is proposed, and implications to teaching, curriculum and research are drawn.