Purpose: Despite the growth of theory and research on young English learners and the widely-recognized importance of critical incidents (CIs) in teachers' professional development (PD), little research has examined young-learner teachers' CIs. This study investigated the impacts of a PD course structured around CIs sharing/discussing on 14 L2 teachers' understanding of teaching young learners. Design: Narratives and semi-structured interviews were used to collect the data. Findings: Data analyses indicated that the teachers reported various interpersonal, pedagogical, and professional CIs, especially problems with learner engagement and the learners who problematized the teachers' knowledge frequently. Additionally, the results of the interviews indicated that the teachers experienced conceptual, practical, and personal-professional changes in their treatment of CIs as a function of participation in the PD course. Value: The study proposes a typology of young-learner teachers' CIs and argues that young-learner teaching requires its specific form of teacher education that fosters in the teachers awareness of emotional, pedagogical, and interpersonal dimensions for teaching this group of learners.