This paper sets out to analyse a case study of adult-children interaction in an educational context from a perspective of argumentation. The paper selects a case in which 3 argumentative discussions are opened and it analyses them with the aim of understanding whether they are fully developed from a point of view of argumentation; or whether they are cut short and why. The paper's focus is not on the children’s individual productions but on the process of interaction. It assumes the pragma-dialectical model of argumentation and the AMT as a theoretical framework. The findings show that none of the discussions opened gets to a concluding stage, either because the teacher shifts the discussion on a different issue, or because the opening stage is not clear, or because the argumentation stage is not adequately developed. These findings contribute to conceptual clarification about how to interpret the role of a teacher.