One-to-one interviewing is one of the most powerful ways of exploring young children’s thinking. While there are a few studies which report the use of this pedagogic strategy by preservice teachers, more are needed. This paper describes an early childhood mathematics education assignment which preservice teachers in a teacher-education college in Ireland undertake with four- and five-year-old children. It presents an analysis of data from a selected sample of 58 preservice teachers’ reflections on their dialogues with individual children. It describes how the preservice teachers engaged with the process of interviewing the children; how they reflected on their interactions with the children; their reflections on the children’s engagement; and their reflections on young children’s mathematics generally. Some dilemmas faced by the participants in the course of the interviewing are highlighted. Findings demonstrate the integrated nature of the knowledge that early childhood teachers need to teach early childhood mathematics effectively. The learning arising from this assignment relates to early childhood mathematics thinking, to the psycho-social nature of learning in early childhood and to pedagogy in early childhood education