A Study of Belumosudil in Children With Chronic Graft Versus Host Disease (schoolROCK)

Inclusion Criteria: * Participant must be 1 to \<18 years of age, at the time the consent/assent is signed. For Phase 1: participant must be 1 to \<12 years of age, at the time the consent/assent is signed. For Phase 2: participant must be 1 to \<18 years of age, at the time the consent/assent is signed. * Participant has undergone an allogeneic HCT * Has active moderate to severe cGVHD, defined using the NIH Consensus diagnosis and staging criteria for which systemic therapy is required * cGVHD is refractory to or has recurred after at least 2 prior lines of systemic treatment * Has received at least two lines of prior systemic therapy for cGVHD, but no more than 5 lines. * If participant receives corticosteroid therapy for cGVHD, the dose must be stable for at least 2 weeks prior to the first dose of the IMP * Has a Lansky-Play (if aged ≤16) or Karnofsky (if aged \>16) performance scale of ≥60 * Body weight of 8 kg and above * Contraceptive use by sexually active male and female should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies * Life expectancy of \>6 months * Participants can take the IMP orally or via a nasogastric tube Exclusion Criteria: * Progressive underlying disease or post-transplant lymphoproliferative disease within 4 weeks prior to the first dose of the IMP. * Diagnosed with another malignancy (other than malignancy for which transplant was performed) within 3 years prior to the first dose of the IMP * History or other evidence of severe illness or any other conditions that would make the participant, in the opinion of the Investigator, unsuitable for the study (such as malabsorption syndromes, active, uncontrolled infections, or poorly controlled psychiatric disease) * Has a forced expiratory volume (in the first second; FEV1) ≤39% or has lung score of 3 * Female participants who are pregnant or breastfeeding * Current treatment with systemic agents for cGVHD (apart from corticosteroids and calcineurin inhibitors), such as ibrutinib, ruxolitinib, sirolimus, mycophenolate (MMF), methotrexate, rituximab, imatinib, extracorporeal photopheresis (ECP) and any investigational cGVHD treatment. Prior treatment with these agents and/or therapy is allowed with a washout of at least 28 days or 5 half-lives, whichever is shorter, prior to the first dose of the IMP * The use of herbal and recreational drugs within 7 days before the start of study intervention * Participant has had previous exposure to belumosudil * Administration of live or live-attenuated vaccines is prohibited within 28 days or 5 elimination half-lives of the respective vaccine, whichever is longer, prior to IMP administration and until study intervention discontinuation * Treatment with any non-GVHD investigational agent, or any investigational device or procedure, within 28 days (or 5 half-lives, whichever is longer) of enrollment, prior to the first dose of the IMP * For Phase 1 only: Administration with strong CYP3A4 inducers is not allowed within 14 days or 5 half-lives (whichever is longer) of the first dose of IMP until the study intervention discontinuation. * For Phase 1 only: PPIs are not allowed within 1 day or 5 half-lives (whichever is longer) of the first dose of IMP and Day 15 of Cycle 1. They can be restarted on Cycle 1 Day 16. * Absolute neutrophil count \<1.0 × 109/L. The use of granulocyte-colony stimulating factor (G-CSF) is not allowed to reach this level during screening * Platelet count \<25 × 109/L. Platelet transfusions are not allowed within 72 hours before hematology screening test. Participants with platelet transfusion refractoriness will be excluded. (Participants who have suboptimal responses to at least 2 transfusions will be considered as platelet transfusion refractory) * Alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) \>3× upper limit of normal (ULN) (\> 5x ULN if abnormalities are due to cGVHD) * Total bilirubin \>1.5 × ULN (\>3 x ULN if Gilbert's syndrome) * Glomerular filtration rate (GFR) \<30 mL/min/1.73 m2 using the revised Bedside Schwartz calculator * Participants with an active viral disease including hepatitis B virus (HBV) and hepatitis C virus (HCV) * Active uncontrolled Cytomegalovirus (CMV) or Epstein-Barr virus (EBV) infection * Known history of human immunodeficiency virus (HIV) * Not suitable for participation, whatever the reason, as judged by the Investigator, including medical or clinical conditions, or participants potentially at risk of noncompliance to study procedures