**Study Overview**: This study is testing a new treatment called **HER2 CAR T cells** for children with a type of brain tumor called **ependymoma**. Ependymoma can sometimes come back or not respond to regular treatments. CAR T cells are a special type of immune cell made from a patient's own blood. Scientists change these cells so they can find and attack the tumor. The study wants to see if this treatment is safe and how well it works. **Key Points**: - **Study Length**: Treatment lasts about 9 to 10 months, but patients are followed for 15 years. - **Visits Needed**: Patients will need multiple visits for infusions and surgery, if applicable. - **Potential Risks**: Includes side effects from chemotherapy and CAR T cell infusion. **Eligibility**: Children aged 1 to 21 with specific health criteria. They should have previously tried standard treatments and be able to understand and agree to the study requirements. Those with certain health conditions, like HIV or bulky tumors, are not eligible.