A Study of Selexipag as Add-On Treatment to Standard of Care in Children With Pulmonary Arterial Hypertension

The purpose of this study is to evaluate whether the addition of selexipag to standard of care treatment delays disease progression in children with Pulmonary Arterial Hypertension (PAH) in comparison to placebo.

Inclusion Criteria: * Participants between greater than or equal to (\>=) 2 and less than (\=9 kilogram (kg) at randomization * Pulmonary arterial hypertension (PAH) diagnosis confirmed by documented historical right heart catheterization (RHC) performed at any time before participant's screening * PAH (World Health Organization \[WHO\] Group 1), including participants with Down syndrome, of the following etiologies: Idiopathic PAH (IPAH); Heritable PAH (HPAH); PAH associated with congenital heart disease (PAH-associated with congenital heart disease \[aCHD\]) (PAH with coincidental CHD \[that is, a small atrial septal defect, ventricular septal defect, or patent ductus arteriosus that does not itself account for the development of elevated PVR\] and if approved by the BCAC) and Post-operative PAH (persisting / recurring/ developing \>=6 months after repair of CHD); Drug or toxin-induced; PAH associated with Human immunodeficiency virus (HIV) * WHO functional class (FC) II and III * Participants treated with at least 1 PAH-specific treatment, example, an Endothelin receptor antagonist (ERA) and/or a Phosphodiesterase type-5 (PDE-5) inhibitor/soluble guanylate cyclase stimulator, provided that the treatment dose(s) has been stable for at least 3 months prior to first dose of study intervention Exclusion Criteria: * PAH due to portal hypertension, schistosomiasis, pulmonary veno-occlusive disease, and/or pulmonary capillary hemangiomatosis * PAH associated with Eisenmenger syndrome * Previous exposure to Uptravi (selexipag) * Known concomitant life-threatening disease with a life expectancy \