GDC-0449 in Treating Young Patients With Medulloblastoma That is Recurrent or Did Not Respond to Previous Treatment

Inclusion Criteria: * Histologically confirmed medulloblastoma, including posterior fossa primitive neuroectodermal tumor (PNET) * Recurrent, progressive, or refractory to standard therapy * No known curative therapy exists * Neurological deficits allowed provided they are stable for ≥ 1 week prior to study entry * No atypical teratoid/rhabdoid tumor or supratentorial PNET * Karnofsky performance status (PS) 60-100% (for patients \> 16 years of age) OR Lansky PS 60-100% (for patients ≤ 16 years of age) * ANC ≥ 1,000/μL\* * Platelet count ≥ 100,000/μL (transfusion independent)\* * Hemoglobin ≥ 8.0 g/dL (RBC transfusion allowed)\* * Creatinine clearance or radioisotope GFR ≥ 70 mL/min OR serum creatinine based on age as follows: * ≤ 0.8 mg/dL (for patients ≤ 5 years of age) * ≤ 1.0 mg/dL (for patients 6 to 10 years of age) * ≤ 1.2 mg/dL (for patients 11 to 15 years of age) * ≤ 1.5 mg/dL (for patients \> 15 years of age) * Total bilirubin ≤ 1.5 times upper limit of normal (ULN) for age * ALT/AST ≤ 2.5 times ULN for age * Serum albumin ≥ 2.5 g/dL * Not pregnant or nursing * Negative pregnancy test * Fertile female patients must use 2 effective methods of contraception during and for 12 months following study treatment * Fertile male patients must use effective barrier contraception during and for 12 months following study treatment * Body surface area \> 0.67 m\^2 and ≤ 2.5 m\^2 * Able to swallow capsules * No malabsorption syndrome or other condition that would interfere with enteral absorption * No history of congestive heart failure * No history of ventricular arrhythmia requiring medication * No uncontrolled hypocalcemia, hypomagnesemia, hyponatremia, or hypokalemia, defined as less than the lower limit of normal despite adequate electrolyte supplementation * No clinically important history of liver disease, including viral hepatitis or cirrhosis * No concurrent clinically significant unrelated systemic illness (e.g., serious infection) or significant cardiac, pulmonary, hepatic, or other organ dysfunction that would compromise the patient's ability to tolerate study treatment or would likely interfere with study procedures or results * NOTE: \* In the absence of bone marrow involvement * Recovered from prior treatment-related toxicity * At least 3 months since prior craniospinal radiotherapy (at doses ≥ 23 Gy) * At least 8 weeks since prior local radiotherapy to primary tumor * At least 2 weeks since prior focal radiotherapy to symptomatic metastatic sites * More than 4 weeks since prior myelosuppressive chemotherapy or immunotherapy (6 weeks for nitrosoureas) * More than 1 week since prior colony-stimulating factors (e.g., filgrastim \[G-CSF\], sargramostim \[GM-CSF\], or erythropoietin) * No other concurrent anticancer or investigational drug therapy * Concurrent dexamethasone allowed provided dosage is stable or decreasing for ≥ 1 week prior to study entry