This study looks at helping kids with high-risk solid tumors live longer without the disease getting worse. **Sirolimus**, also called rapamycin, is a drug that stops the immune system from attacking. It is used with **metronomic chemotherapy**—a low-dose, daily cancer treatment. This study checks if this combo works better than just watching the patients after their usual treatment. Children can join if they are between 1 to 30 years old and have certain types of cancer. They must be in complete remission or have minimal signs of cancer after their initial treatment. Sirolimus can help prevent cancer cells from growing by stopping certain cell activities. There are three groups in the study: a group with new high-risk tumors, one with recurring tumors, and a past group for comparison. The study lasts two years and involves continuous sirolimus and chemotherapy.

• Participants should have stable health and meet specific blood, liver, and kidney function levels.
• There are restrictions on certain medications and participants must not have active infections.
• Pregnant or breastfeeding women cannot join, and participants must agree to use birth control during and after the study.