This paper investigated how production task combined with teacher feedback (in the form of recasts) affects child second language development, and the effects of task complexity on their production performance. 92 child learners of English in three intact classes were assigned to three tasks of different complexity (simple, +complex, ++complex). In response to errors in the use of the present third person singular verb forms, participants received the feedback of recasts. L2 development was measured through oral production and written tests. Results show that recasts are effective for learning the target form, and there is a negative correlation between task complexity and child learners' acquisition of linguistic target in their written production, but no significant correlation was found between task complexity and participants' oral production. The results confirm Skehan's limited attention capacity prediction, and can be explained from the cognitive development level of the participants and their English learning habits. Implications for pedagogy and future research are discussed.