This study tests a special treatment for children with a kind of brain tumor called ependymoma. Ependymoma is a tumor that can come back or not respond to regular treatments. The treatment uses HER2 CAR T cells, where CAR stands for Chimeric Antigen Receptor. This is a type of immune therapy where doctors take a patient's own blood cells, change them to fight the tumor, and put them back into the patient's body. The goal is for these T cells to find and kill the tumor cells.

The study has two parts: Phase I and a Surgical Study. In Phase I, safety and how well these cells can be given to patients in different places are checked. The Surgical Study looks at how these cells work in patients needing surgery. Kids in this study will go through different steps over several months. They need to be checked to make sure their organs are healthy and they don't have certain other health issues.

• The study treatment lasts about 9-10 months, but follow-ups continue for 15 years.
• Participants may need to travel for multiple treatment sessions and check-ups.
• This is an investigational treatment, so there may be risks and unknowns involved.