Study of Nifurtimox to Treat Refractory or Relapsed Neuroblastoma or Medulloblastoma

Inclusion Criteria: * Age: 0-21 years at the time of diagnosis. * Diagnosis: Histologic verification at either the time of original diagnosis or relapse of neuroblastoma or medulloblastoma. * Disease Status: Refractory or first or multiple relapsed neuroblastoma, or medulloblastoma that has relapsed after, or is refractory to, a chemotherapy-containing treatment regimen. * Measurable disease, including at least one of the following: * Measurable tumor by CT or MRI * For neuroblastoma patients only, a positive MIBG (MIBG not required if subject's neuroblastoma is previously determined to not uptake MIBG), abnormal urinary catecholamine levels, or positive bone marrow biopsy/aspirate. * For medulloblastoma patients only, positive CSF cytology * Current disease state must be one for which there is currently no known curative therapy. * A negative urine pregnancy test is required for female participants of child bearing potential (≥13 years of age). * Organ Function Requirements Patients without bone marrow metastases must have an ANC \> 500/μl and platelet count \>50,000/μl. * Patients must have adequate liver function as defined by AST or ALT \<10x normal * Informed Consent: All patients and/or legal guardians must sign informed written consent. Assent, when appropriate, will be obtained according to institutional guidelines Exclusion Criteria: * Life expectancy \<2 months or Lansky score \<50% * Investigational Drugs: Patients who are currently receiving another investigational drug are excluded from participation. * Anti-cancer Agents: Patients who are currently receiving other anticancer agents are not eligible. Patients must have fully recovered from the effects of prior chemotherapy, generally at least 3 weeks from the most recent administration (6 weeks for nitrosoureas). * Infection: Patients who have an uncontrolled infection are not eligible until the infection is judged to be well controlled. * Patients who, in the opinion of the investigator, may not be able to comply with the safety monitoring requirements of the study, or in whom compliance is likely to be suboptimal, should be excluded. Compensation for travel related expenses may be available