Final answer:
The symptoms described can be indicative of neuroblastoma, a type of childhood cancer arising from immature nerve cells, characterized by an abdominal mass and possible mediastinal mass.
Step-by-step explanation:
The question concerns the diagnosis of a 3-year-old child presenting with a flank mass showing calcifications and a posterior mediastinal mass. These symptoms could indicate a pediatric malignancy, such as neuroblastoma, which is a common cancer in children that develops from immature nerve cells in the body and often presents with an abdominal mass, which could appear as calcifications on imaging. Neuroblastomas can also occur in the chest, presenting as a posterior mediastinal mass. The combination of these findings is suggestive of this diagnosis, but a definitive diagnosis would require further medical evaluation including biopsy, imaging studies, and laboratory tests.