Additional Study Information: The purpose of this study is to find the best dose of a drug called MTX110 and a contrast agent called gadolinium that can be given safely in children with newly diagnosed diffuse midline gliomas, which is a kind of brain tumor. All patients enrolled in the study will receive infusion of MTX110 and Gadolinium delivered with a pump directly into the tumor over 9-11 days. This is called a cycle chemotherapy and will be repeated one more time. The reason of using an experimental method of directly infusing the drug into the tumor is because the blood-brain barrier (BBB) does not allow drugs from reaching the tumor. By using this new method of infusion of drugs, the drugs can now pass the BBB and reach the tumor successfully. The BBB can also retain drugs in the brain and therefore significantly reduce other side effects experienced by the patient. The study is composed of the following periods: Screening, Treatment and Follow-up.