The overall goal of this study is to find out if a reduced dose of radiation to the whole brain and spine and less chemotherapy can be given to children and young adults with WNT subtype medulloblastoma without increasing the risk of the tumor coming back. The combination of a reduction in radiation therapy and chemotherapy is experimental. Other goals of this study are: (1) better understand the biology of WNT subtype Medulloblastoma; (2) closely examine the brain function as it relates to thinking and learning in children with WNT subtype Medulloblastoma after they receive treatment on this study.