This study looks at how well a drug called selumetinib works when given to children and young adults with LGG. The study doctors want to see if selumetinib works as well as CV for getting rid of or shrinking LGGs and stopping them from coming back. They also want to know if this treatment will improve quality of life for subjects who take it. The overall goal of this study is to see if selumetinib works just as well as the standard treatment of CV for subjects with LGG.