This study investigated the effects of task complexity on child learners' second language (L2) gains, the relationship between aptitude and L2 development, and the extent to which task complexity influences this relationship when recasts are provided. Sixty child EFL learners were assigned to two experimental groups. During the treatment, one group completed simple information transmission tasks, whereas the other group performed complex decision-making tasks. In response to errors in the use of the present third person singular verb forms, participants received recasts. L2 development was measured through oral production, written production, and elicited imitation tests. Aptitude was assessed through LLAMA D, LLAMA E, and LLAMA F. Less cognitively demanding tasks were more beneficial. Participants' performance on LLAMA E predicted L2 gains measured through elicited imitation, and their LLAMA D scores predicted development measured through the oral and written production tests under complex task conditions.