Additional Study Information: Children and young adults with diffuse midline glioma will be treated with Focused Ultrasound (FUS) with DEFINITY microbubbles and neuro-navigator-controlled sonication. The overall goal of this study is to see if the study treatment plan will improve the outcome of participants with diffuse midline glioma, for which no known cure exists, and determine whether there are any clinical benefits of the study treatment. Children with progressive diffuse midline gliomas (DMGs) have very poor outcomes and have very limited options. One of the main obstacles in the effectiveness of medical therapy in brain tumors is the presence of the Blood Brain Barrier (BBB). The BBB is a border between your brain and spine and the rest of your body that separates circulating blood from the brain and its fluid. The presence of the BBB prevents many drugs from reaching the brain and any tumors within it. However, studies have shown that the use of a focused ultrasound (FUS) along with intravenously administered DEFINITY microbubbles creates a targeted drug delivery system that may allow the drug to pass through the BBB and treat the tumor. Focused ultrasound is a non-invasive technique (meaning it does not cut the skin) used to open the blood-brain barrier temporarily along with the use of microbubbles. Once the BBB is open, subjects will be given Etoposide (an oral medication).