Collaborative writing (CW) tasks have been shown to generate interactional moves conducive to L2 learning. However, recent studies have demonstrated that learner-related variables, including interaction mindset (IM), willingness to communicate (WTC), and proficiency may explain the extent to which L2 learners notice errors or interact in these tasks. This study explores the impact of these individual differences on the interactional behaviours of child EFL learners, a population where this topic has been barely investigated. Fifty-seven children (ages 11–12) took part in the study and completed a CW task. Their proficiency was measured through a standardized test, and IM and WTC were measured from a trait-like and situation-specific perspective using a questionnaire and focal interviews. The dependent variable, L1/L2 oral task-based production, was quantified in words and turns. The multiple regression analyses failed to show any significant contribution of IM and WTC to learners’ L1/L2 use, while proficiency significantly impacted their L1 use: more proficiency led to less L1 use. Participants’ responses from the post-task interview highlighted the situational nature of IM and WTC. These findings shed light on the effects of child individual variables on CW tasks and suggest that future studies can test whether amount of learner talk in L1/L2 as mediated by proficiency determines ultimate learning.