Previous research suggests that child HSs’ performance in offline linguistic tasks is typically worse than their age-matched monolingual peers and is modulated by linguistic and child-level factors. This study examined the comprehension and production of three Mandarin non-canonical structures in 5- to 9-year-old Mandarin–English heritage children and Mandarin-speaking monolingual children, including an online processing task. Results showed that heritage children had different performance in production and offline comprehension across structures compared to monolinguals. In online processing, they showed sensitivity to different cues similarly to monolinguals but took longer to revise initial misinterpretations. Within heritage children, we found that presence of morphosyntactic cues facilitated performance across tasks while cross-linguistic influence was only identified in production and offline comprehension but not in online processing. Additionally, input quantity predicted their production and offline comprehension accuracy of non-canonical structures, whereas age modulated their production. Lastly, online processing was not modulated by age nor input.