DISEASE CHARACTERISTICS: - Diagnosis of 1 of the following: - Histologically confirmed malignant solid tumor at original diagnosis or relapse - Neuroblastoma - Rhabdomyosarcoma (RMS) - Osteosarcoma - Ewing sarcoma/peripheral primitive neuroectodermal tumor - Non-RMS soft tissue sarcoma - Hepatoblastoma - Malignant germ cell tumor - Wilms tumor - Histologically confirmed leukemia recurrent or refractory to ≥ 2 prior induction or treatment regimens (must not be known to be refractory to red blood cell or platelet transfusions) - Acute lymphoblastic leukemia - > 25% blasts in the bone marrow (M3 bone marrow), excluding known CNS disease - Acute myeloid leukemia - ≥ 5 % blasts in the bone marrow (M2/M3 bone marrow), excluding known CNS disease - Radiographically measurable disease for solid tumors - Patients with neuroblastoma who do not have measurable disease but have iodine-123 metaiodobenzylguanidine (¹²³ I-MIBG)-positive lesions allowed - None of the following qualify as measurable disease: - Malignant fluid collections (e.g., ascites, pleural effusions) - Bone marrow infiltration - Lesions detected by nuclear medicine studies (e.g., bone, gallium, or PET scans) - Elevated tumor markers in plasma or cerebrospinal fluid - Previously irradiated lesions that have not demonstrated clear progression after radiotherapy - No CNS disease (leukemia patients) PATIENT CHARACTERISTICS: - ECOG performance score (PS) 0-2 - Karnofsky PS 50-100% (for patients > 16 years of age) - Lansky PS 50-100% (for patients ≤ 16 years of age) - ANC ≥ 1,000/mm³ - ANC ≥ 750/mm³ (solid tumor and known bone marrow metastatic disease) - Transfusion allowed - Hemoglobin ≥ 8.0 g/dL (RBC transfusion allowed) - Platelet count ≥ 100,000/mm³ (transfusion independent, > 7 days since platelet transfusion ) - Platelet count ≥ 50,000/mm³ (solid tumor and known bone marrow metastatic disease) - Transfusion allowed - Not refractory to RBC or platelet transfusions - Creatinine clearance or radioisotope GFR ≥ 70 mL/min OR a serum creatinine based on age and/or gender as follows: - 0.6 mg/dL (1 to < 2 years of age) - 0.8 mg/dL (2 to < 6 years of age) - 1.0 mg/dL (6 to < 10 years of age) - 1.2 mg/dL (10 to < 13 years of age) - 1.5 mg/dL (male) and 1.4 mg/dL (female) (13 to < 16 years of age) - 1.7 mg/dL (male) and 1.4 mg/dL (female) (≥ 16 years of age) - Total bilirubin ≤ 1.5 times upper limit of normal (ULN) - ALT ≤ 5.0 times ULN - Serum albumin ≥ 2 g/dL - Not pregnant or nursing - Negative pregnancy test - Fertile patients must use effective contraception during and for ≥ 3 months after completion of study therapy - Not unable to swallow capsules - No uncontrolled infection - No patient who, in the opinion of the investigator, may not be able to comply with the safety monitoring requirements of the study PRIOR CONCURRENT THERAPY: - See Disease Characteristics - Fully recovered from all prior chemotherapy, immunotherapy, or radiotherapy - More than 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosourea) (solid tumor patients) - At least 14 days since prior cytotoxic therapy (leukemia patients) - Patients who relapse during standard maintenance therapy are not required to wait 14 days - At least 24 hours since prior cytoreduction with hydroxyurea - More than 7 days since prior growth factors (14 days for pegfilgrastim [Neulasta]) - At least 7 days since prior biologic agent (antineoplastic agent) - At least 3 half-lives since prior monoclonal antibody therapy - At least 2 weeks since prior local palliative radiotherapy (small port) - At least 6 weeks since prior therapeutic doses of ¹²³ I-MIBG or other substantial bone marrow irradiation - At least 6 months since prior craniospinal radiotherapy, radiotherapy to ≥ 50% of the pelvis, or total-body irradiation - At least 3 months since stem cell transplantation with no evidence of active graft-vs-host disease - Concurrent corticosteroids allowed provided patient is on a stable or decreasing dose for ≥ 7 days before study enrollment - No other concurrent investigational drugs - No other concurrent anticancer agents, including chemotherapy, radiotherapy, or immunomodulating agents - No concurrent daily benzodiazepine therapy - No concurrent P-glycoprotein substrates (e.g., digoxin, cyclosporine, tacrolimus, or sirolimus)