Research on collaborative tasks with young learners has, to date, paid little attention to individual differences (IDs), such as learners’ engagement in a task and their willingness to communicate (WTC) in an L2 (Munezane, 2013). These IDs should be taken into account, given their significant role in second language acquisition (SLA) (Dörnyei & Ryan, 2015), and in the assessment of learners’ performance (Pawlak, 2017). The aim of this study was to analyse the interplay between (1) WTC, (2) engagement in collaborative tasks, operationalised as interaction mindset (IM), and (3) the language analytical ability of young learners (YLs) of English as Foreign Language (EFL), in their collaborative writing task performance. The participants were 67 young EFL learners (11–12 years old) from a semi-private school in Spain. Researcher-selected dyads of similar language competence performed seven written collaborative picture-prompted narration tasks over the span of eight weeks. Questionnaires targeting learners’ IDs were gathered prior to the onset of the study and again once the tasks were completed. Data were analysed in order to explore the impact of YLs’ IDs on the quality and accuracy of the written product resulting from their interaction in week one (pre-test). The results indicate that the relationship between the learners’ IDs and their written collaborative performance should not be overlooked, as children with better language analytical ability, and who were more engaged in the task, seem to perform better. These findings support the importance of individual factors affecting L2 acquisition in collaborative settings.