DISEASE CHARACTERISTICS: Histologically confirmed solid tumor that is refractory to standard therapy or for which no effective therapy exists Brainstem gliomas allowed without histologic diagnosis Brain tumors allowed provided not receiving anticonvulsants Strata 2 and 3: No bone marrow involvement

PATIENT CHARACTERISTICS: Age: 1 to 21 Performance status: Karnofsky 50-100% for patients over 10 years of age OR Lansky 50-100% for patients 10 years of age and under Life expectancy: At least 8 weeks Hematopoietic: Absolute neutrophil count at least 1,000/mm3 Hemoglobin at least 8.0 g/dL Platelet count at least 100,000/mm3 Hepatic: Albumin at least 2.5 g/dL Bilirubin no greater than 1.5 mg/dL SGPT no greater than 2 times upper limit of normal Renal: Creatinine normal for age OR GFR normal for age Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception If CNS tumor, neurologic deficits relatively stable for at least 2 weeks Must be at least third percentile weight for height No concurrent significant systemic illness (e.g., infection, fever, mucositis, severe anorexia, and severe malnutrition) No uncontrolled infections No evidence of graft versus host disease

PRIOR CONCURRENT THERAPY: Biologic therapy: At least 1 week since prior biologic therapy and recovered At least 6 months since prior autologous or allogeneic bone marrow transplantation without total body irradiation (TBI) At least 1 week since prior growth factors Strata 2 and 3: No prior stem cell transplantation (with or without TBI) Chemotherapy: At least 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosoureas) and recovered At least 1 week since prior antineoplastic agents No other concurrent chemotherapy Strata 2 and 3: No more than 2 prior chemotherapy regimens (single or multiagent) Endocrine therapy: If receiving dexamethasone for CNS tumors, must be on stable or decreasing dose for at least 2 weeks Radiotherapy: At least 2 weeks since prior local radiation (small port) At least 6 months since prior craniospinal radiation At least 6 months since prior radiation to at least 50% of pelvis At least 6 weeks since prior substantial bone marrow radiation Recovered from prior radiotherapy Strata 2 and 3: No prior central axis radiotherapy Surgery: Not specified Other: No other concurrent investigational agents No other concurrent anticancer therapy No concurrent anticonvulsants