This phase III trial studies if selumetinib works just as well as the standard treatment with carboplatin/vincristine (CV) for subjects with NF1-associated low grade glioma (LGG), and to see if selumetinib is better than CV in improving vision in subjects with LGG of the optic pathway (vision nerves). Selumetinib is a drug that works by blocking some enzymes that low-grade glioma tumor cells need for their growth. This results in killing tumor cells. Drugs used as chemotherapy, such as carboplatin and vincristine, 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. It is not yet known whether selumetinib works better in treating patients with NF1-associated low-grade glioma compared to standard therapy with carboplatin and vincristine.