This study investigates the safety and best dose of an investigational medication given with chemotherapy drugs, temozolomide and irinotecan, in children with solid tumors that have returned or not responded to treatment. Solid tumors are growths of abnormal cells that can occur in various parts of the body. This study focuses on finding the right dose of the investigational medication, which is given through an IV, and understanding its effects on the body.

Participants will receive the investigational medication through an intravenous infusion over 30 minutes on specific days of a 21-day cycle, along with the chemotherapy drugs. The study will monitor any side effects and how the body absorbs the medication. This process helps researchers learn more about the investigational medication's safety and how it works when combined with other treatments.

• Who can participate: Children with solid tumors that have come back or not responded to treatment can participate. They must have a confirmed diagnosis and either measurable or evaluable disease. Participants should not be pregnant, breastfeeding, or on certain medications. They must have fully recovered from the acute toxic effects of all prior anti-cancer therapy and meet the minimum duration from prior therapy.
• Study details: Participants will receive an investigational medication through an IV, along with chemotherapy drugs, on specific days of a cycle. A placebo will not be used in this study. Participants will not be paid for their participation.