Driven by the rising popularity of chatbots such as ChatGPT, there is a budding line of research proposing guidelines for chatbot design, both in general and specifically for digital education. Nevertheless, few researchers have focused on providing conceptual tools to frame the chatbot design process itself. In this paper, we present a model to guide the design of educational chatbots. Our model aims to structure participatory design sessions in which different stakeholders (educators, developers, and learners) collaborate in the ideation of educational chatbots. To validate our model, we conducted an illustrative study in which 25 software design students took part in a simulated participatory design session. Students were divided into eight groups, assigned the role of one of the different stakeholders, and instructed to use our model. The results of our qualitative analysis suggest that our model helped structure the design process and align the contributions of the various stakeholders.