Valproic Acid in Treating Young Patients With Recurrent or Refractory Solid Tumors or CNS Tumors

DISEASE CHARACTERISTICS: * Histologically confirmed\* malignant solid tumor, including CNS tumors, at original diagnosis or relapse * Recurrent or refractory disease NOTE: \*Histologic confirmation not required for intrinsic brain stem or optic pathway tumors * Measurable or evaluable disease, defined by 1 of the following criteria: * Any unidimensionally measurable lesion ≥ 10 mm by standard MRI or CT scan for either solid or CNS tumors * At least 1 nonmeasurable lesion that is evaluable by nuclear medicine, immunocytochemistry, tumor markers, cerebrospinal fluid cytology, or other reliable measures * No known curative therapy exists * No documented tumor involvement in the bone marrow PATIENT CHARACTERISTICS: Age * 2 to 21 Performance status\* * Lansky 50-100% (for patients ≤ 10 years of age) * Karnofsky 50-100% (for patients \> 10 years of age) Life expectancy * Not specified Hematopoietic * Absolute neutrophil count ≥ 1,000/mm\^3 * Platelet count ≥ 100,000/mm\^3 (transfusion independent) * Hemoglobin ≥ 8.0 g/dL (transfusions allowed) Hepatic * Bilirubin ≤ 1.5 times upper limit of normal (ULN) * ALT ≤ 110 (ULN for this study is 45 U/L) * Albumin ≥ 2 g/dL Renal * Creatinine clearance or radioisotope glomerular filtration rate ≥ 70 mL/min OR * Creatinine based on age as follows: * No greater than 0.8 mg/dL (for patients ≤ 5 years of age) * No greater than 1.0 mg/dL (for patients 6 to 10 years of age) * No greater than 1.2 mg/dL (for patients 11 t…