Additional Study Information: This is a phase I, multicenter, dose escalation study of MK-1775 in combination with radiation. The study treatment is considered experimental because it is not approved by the United States (US) Food and Drug Administration (FDA) for treating children and young adults with newly diagnosed Diffuse Intrinsic Pontine Glioma (DIPG). MK-1775 is an oral selective Wee1 kinase inhibitor that works by inhibiting cancer cell growth. In this study, MK-1775 will be administered in combination with standard radiation therapy regimen. The purpose of this study is to evaluate the toxicities and clinical activity of MK-1775 in children with newly diagnosed with DIPG. Radiation therapy will be given at the current standard dose once daily Monday through Friday for a total of 30 doses over 6 weeks. K-1775 will also be given based on the assigned dose level once daily Monday through Friday after that days radiation therapy, for a total of 30 doses over 6 weeks. Disease assessment will be completed before beginning treatment, before starting a new cycle, at the end of treatment, at post-treatment follow-up visits, and at the end of the study. During every cycle, blood samples will be collected to determine how much MK-1775 is in the blood (pharmacokinetics). The blood tests will help us to better learn how to use this drug for children who may receive it in the future.