<p>When learning mathematics, children interact with each other, adults, and manipulatives in various ways. One form of social interaction especially suitable for enhancing learners’ mathematical competencies is mathematical argumentation. To make the best use of those fruitful learning opportunities, professionals need to identify the mathematical content at hand as well as children’s argumentative approaches. This qualitative study with five early childhood professionals offers insight into early education professionals’ situational observing and perceiving of different aspects of explorative mathematical argumentation in a geometrical learning situation. The results show a wide range of professionals’ perceptions of adult–child interactions in a mathematical argumentation process. Multiprofessional evaluation of mathematical argumentation processes is discussed as a possible consequence.</p>