RATIONALE: Drugs used in chemotherapy, such as topotecan, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Colony-stimulating factors, such as G-CSF, may increase the number of immune cells found in bone marrow or peripheral blood and may help the immune system recover from the side effects of chemotherapy. Radiation therapy uses high-energy x-rays to kill tumor cells. Topotecan may make tumor cells more sensitive to radiation therapy . Giving topotecan and G-CSF together with radiation therapy may be an effective treatment for brain stem glioma. PURPOSE: This phase I/II trial is studying the side effects and best dose of topotecan when given together with G-CSF and radiation therapy and to see how well they work in treating young patients with newly diagnosed brain stem glioma.