This study is for kids aged **1 to less than 18** years who have chronic graft-versus-host disease (cGVHD), a condition where the transplanted cells attack the body. The study will see if a medicine called **belumosudil** can help those who haven’t gotten better from other treatments. There are two phases: **Phase 1** will find the right dose for younger children, and **Phase 2** will check how safe and effective the medicine is. **Key Points to Consider:** - **Study Length:** The study lasts until each child finishes treatment or up to **3 years** after the last child joins. - **Participation:** Includes a screening period of up to **4 weeks**, treatment, and a **30-day** safety follow-up. - **Eligibility:** Must have tried at least **2 other treatments** for cGVHD and be able to take medicine by mouth or tube. Participants must not have serious illnesses or other conditions that could interfere with the study. There are specific rules about other medicines and treatments that participants can use. Parents or guardians must give permission for their child to join.