Research into students' understanding of complex systems typically ignores young children because of misinterpretations of young children's competencies. Furthermore, studies that do recognize young children's competencies tend to focus on what children can do in isolation. As an alternative, we propose an approach to designing for young children that is grounded in the notion of the Zone of Proximal Development (Vygotsky 1978) and leverages Activity Theory to design learning environments. In order to highlight the benefits of this approach, we describe our process for using Activity Theory to inform the design of new software and curricula in a way that is productive for young children to learn concepts that we might have previously considered to be "developmentally inappropriate". As an illuminative example, we then present a discussion of the design of the BeeSign simulation software and accompanying curriculum which specifically designed from an Activity Theory perspective to engage young children in learning about complex systems (Danish 2009a, b). Furthermore, to illustrate the benefits of this approach, we will present findings from a new study where 40 first- and second-grade students participated in the BeeSign curriculum to learn about how honeybees collect nectar from a complex systems perspective. We conclude with some practical suggestions for how such an approach to using Activity Theory for research and design might be adopted by other science educators and designers.