We report the successful treatment of hepatoblastoma in a child with hypoplastic left heart syndrome and mosaic trisomy 7. Severe cardiac comorbidities compromised venous access, and the risk of chemotherapy-related toxicity posed significant challenges. A multidisciplinary team opted for cisplatin monotherapy and tailored surgical timing. Despite anatomical and genetic complexity, the patient achieved complete remission and long-term survival. At age 13.5, he remains tumor-free with good Fontan circulation. This case illustrates the feasibility of curative cancer therapy in children with single-ventricle physiology when guided by individualized and collaborative decision-making.