The study compares the effectiveness of two communicative approaches to grammar teaching, Processing Instruction (PI) and Production-Based instruction with collaborative tasks (PB), on the acquisition of past tense verb morphology by child heritage learners of Greek. Fourteen child learners of Greek, students at a heritage language school in Hungary, participated in the study. Half of them received PI instruction, which focuses only on the input, providing structured-input activities, and the other half received PB instruction, which focuses on the production of output with meaningful collaborative tasks. Both teaching interventions included a short explicit grammar teaching module. A pre-/post-test design was employed, which consisted of two tasks: a written production task and a sentence repetition task. Results revealed significant gains for both PI and PB instruction. However, both teaching interventions yielded better results in the written production task than in the more cognitively demanding sentence repetition task. Additionally, children who had little or no previous exposure to Greek seem to have profited more in the more cognitively demanding task, irrespective of the type of teaching intervention.