A 2-year-old boy presents with a large abdominal mass. Imaging reveals a large, unifocal liver tumor in the right lobe, and his α-fetoprotein level is greater than 100,000 ng/mL. A biopsy confirms hepatoblastoma with mixed epithelial and mesenchymal elements. The surgical team deems the tumor unresectable at diagnosis. What is the most appropriate initial management step?