A 7-year-old child presents with a 3-month history of headache, nausea, and recent onset of gait disturbances. MRI of the brain reveals a midline cerebellar mass that is hyperdense on CT, shows restricted diffusion on diffusion-weighted imaging (DWI), and demonstrates heterogeneous enhancement after contrast administration. Additionally, there is evidence of metastatic spread to the spinal cord. Which of the following is the most likely diagnosis?