Engagement with language (EWL) refers to learners’ agency over language and includes cognitive, social and affective dimensions. In collaborative writing (CW) tasks, learners are expected to display high EWL conducive to L2 learning. However, recent research with young learners has shown great variability in their patterns of interaction and metalinguistic discussions while working on such tasks repeatedly, questioning the efficacy of task repetition (TR), a topic that has been overlooked in the EWL literature. This study fills this gap and explores the fluctuations in EWL of young learners of English as a foreign language. Nine pairs in their sixth year of primary school (ages 11–12) in Spain completed different versions of a picture-narration CW task over six weeks. The observation of their interaction and a focus group interview made it possible to obtain a holistic understanding of EWL, including language-related episodes (LREs; i.e., cognitive engagement), patterns of interaction (i.e., social engagement) and learners’ emotions and number of turns (i.e., affective engagement). The results show that while social engagement remained more stable over time, their cognitive engagement and some measures of affective engagement decreased as a consequence of TR, as indicated by the analysis of LREs and turns. However, their views about the task were favourable in general. Moreover, the task content, work distribution and the pre-existing relationship with their partner appear to have contributed to EWL.