A Study of Ifinatamab Deruxtecan in Pediatric Participants With Relapsed or Refractory Solid Tumors (MK-9999-01D/LIGHTBEAM-U01)

Researchers are looking for new ways to treat children with relapsed or refractory solid tumors: * Relapsed means the cancer came back after treatment * Refractory means the cancer did not respond (get smaller or go away) to treatment * Solid tumors are cancers mostly in body organs and tissues, not in the blood or other body liquids The study treatment I-DXd (also known as MK-2400 or ifinatamab deruxtecan) is an antibody-drug conjugate (ADC). An ADC attaches to a protein on cancer cells and delivers treatment to destroy those cells. The goals of this study are to learn: * About the safety of I-DXd and if children younger than 12 years old tolerate it * How many children who receive I-DXd have the cancer get smaller or go away

The main inclusion criteria include but are not limited to the following: * In Part 1, participant has recurrent or relapsed, refractory solid tumors (excluding primary central nervous system (CNS)); and in Part 2, participant has recurrent or relapsed, refractory and histologically confirmed diagnosis of osteosarcoma (OST), neuroblastoma (NBL), rhabdomyosarcoma (RMS), or Wilms tumor (WT). All participants must meet the following criteria: Has documented radiological disease progression after at least 1 line of prior therapy in the locally advanced/metastatic setting and who has no satisfactory alternative treatment option (ie, is ineligible for other standard treatment regimens). * Is an individual of any sex/gender, ≥1 month to \