This randomized phase II trial studies how well giving combination chemotherapy with or without sodium thiosulfate works in preventing low platelet count while treating patients with malignant brain tumors. Drugs used in chemotherapy, such as carboplatin, cyclophosphamide, and etoposide phosphate, work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Sodium thiosulfate may prevent low platelet counts in patients receiving chemotherapy. It is not yet known whether combination chemotherapy is more effective with or without sodium thiosulfate in preventing low platelet count during treatment for brain tumors.