This phase 3 trial compares the effect of selumetinib versus the standard of care treatment with carboplatin and vincristine (CV) in treating patients with newly diagnosed or previously untreated low-grade glioma (LGG) that does not have a genetic abnormality called BRAFV600E mutation and is not associated with systemic neurofibromatosis type 1. Selumetinib works by blocking some of the enzymes needed for cell growth and may kill tumor cells. Carboplatin and vincristine are chemotherapy drugs that work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. The overall goal of this study is to see if selumetinib works just as well as the standard treatment of CV for patients with LGG. Another goal of this study is to compare the effects of selumetinib versus CV in subjects with LGG to find out which is better. Additionally, this trial will also examine if treatment with selumetinib improves the quality of life for subjects who take it.