1. A 5-year-old boy presents with an abdominal mass. He undergoes a distal pancreatectomy. Gross examination reveals a well-circumscribed, lobulated, tan-yellow mass. Microscopically, the tumor is composed of solid nests and sheets of cells with small, round, centrally located nuclei and modest amounts of pale cytoplasm. There are numerous small lumens present at the apical aspect of the cells. Scattered throughout the tumor are nests of larger cells with abundant eosinophilic cytoplasm and larger, elongated nuclei. What is the most likely diagnosis?