The acquisition of voice in Greek remains understudied, especially in heritage populations. Voice in Greek poses a challenging acquisition task for children due to its syncretism, marking various verb classes as well as passives. The present study explores the acquisition of anticausatives, reflexives, and passives in 6-to-8-year-old monolingual Greek and Greek-German bilingual children with Greek as their heritage language. This age coincides with the immersion of children in the education system. A Sentence Picture Matching task tested participants' interpretation preferences when ambiguity arises in reflexives and (optionally) (un)marked anticausatives (reflexive/anticausative vs. passive interpretation); it also tested accuracy in passives compared to actives-transitives (passive vs. active interpretation). All children exhibited adult-like performance in reflexives. In anticausatives, despite their lower performance, they were qualitatively similar to adults, exhibiting knowledge of verb classes. Children's preferences were predicted by age (active-morphology/ACT) and vocabulary (non-active/NACT), confirming the role of the mental lexicon in the acquisition of NACT anticausatives. The only difference between bilingual and monolingual children was in optionally (un)marked anticausatives, both in ACT and NACT, that was marginally predicted by age and grammar production respectively, suggesting that optionality affects bilingual more than monolingual children. In passives, children were less accurate than adults despite ceiling performance in actives-transitives. Bilingual children performed less well than monolingual children and performance was predicted by grammar and exposure. These findings reflect the difficulties of monolingual children with passives and their late acquisition which further affects bilingual development due to reduced input in heritage Greek.