This study is testing a drug called codrituzumab for children and young adults with solid tumors that didn't respond to other treatments or came back after treatment. Researchers want to see if codrituzumab is safe and how it acts in the body. A solid tumor is a mass of cancer cells that can grow in different body parts. Codrituzumab targets a protein called GPC3 found in some tumors. Participants must be aged 12 months to under 22 years and have a tumor that shows GPC3. The study will measure how well patients can function and if they have any brain issues. Pregnant individuals cannot participate, and participants must agree to use effective birth control. They should not be on other cancer treatments and must have stopped certain therapies for a specific time. The study lasts until the end of the trial, with follow-ups to check health and any side effects.

• The study is for those aged 1 to under 22 with certain solid tumors.
• Participants must have a tumor showing the GPC3 protein.
• They must stop other treatments for a specific time before joining.