Collaborative writing (CW) tasks require heightened Engagement with Language (EWL), as their learning potential has been shown to be mediated by the number of language-related episodes (LREs) or learners' patterns of interaction. Recent research on CW with young populations has investigated different dimensions of EWL, but a holistic view of the three main aspects of EWL -- social, cognitive and affective -- is inexistent. This study tries to fill this gap and explores EWL in a group of 69 young Spanish EFL learners (ages 11-12) who completed a picture narration writing task in pairs. The three dimensions of EWL were analysed qualitatively and quantitatively by means of number of turns and LREs, patterns of interaction, and a semi-structured post-task interview. The results indicated that these learners were capable of correctly resolving different linguistic issues, but the extent to which they elaborated on these discussions was low, regardless of their pattern of interaction, which was either cooperative or collaborative most times. A collaborative pattern seemed to lead to more affective engagement and LREs, indicating certain interrelation between these three dimensions, although encouragement and support were seldom verbalised.