This study investigates the effects of an investigational antibody in treating children with solid tumors that have come back or do not respond to treatment. The purpose of this study is to find the best dose and understand the side effects of the investigational antibody. The study focuses on children with recurrent or refractory solid tumors, which means tumors that have returned after treatment or do not respond to treatment. It also looks at how well the investigational antibody works against osteosarcoma, a type of bone cancer.

Participants will receive the investigational antibody as an infusion into their veins every 14 days. The study will monitor for side effects and how the body processes the antibody, which is known as pharmacokinetics. This study will also check if the investigational antibody helps control the disease or shrink tumors in patients with osteosarcoma. Participants will not be paid for their participation.

• Who can participate: Children with recurrent or refractory solid tumors, excluding those with CNS tumors, can participate. They must have verified malignancy and measurable or evaluable disease, have recovered from prior treatments, and not have used the investigational antibody before.
• Study details: Participants will receive the investigational antibody through an intravenous infusion every 14 days. They will be monitored for side effects and how their body processes the antibody. The study aims to find the best dose and understand the side effects.