A 6-year-old boy is taken to a paediatrician because he has developed pain in his right hip. On physical examination, the doctor feels a large mass near the iliac crest. Plain X-ray films demonstrate a large lytic lesion of the ilium. MRI studies show that the tumor appears to arise in the bone, but extends into the adjacent soft tissues. A large incisional biopsy demonstrates a tumor composed of sheets of small, round, blue cells. Which of the following is the most likely diagnosis?