The purpose of this study is to find out whether an early three-day course of an oral steroid medication (dexamethasone) can improve the physical and mental recovery and wellbeing for children with Sydenham's chorea. Sydenham's chorea is a condition that impacts approximately 12% of children with acute rheumatic fever. It is caused by inflammation in the brain following an abnormal immune response to Group A streptococcus bacterial infection. Sydenham's chorea is a movement disorder that causes children's faces, hands, and feet to move quickly and uncontrollably, and can also affect mood and concentration. The physical recovery from Sydenham's chorea can take two to six months but the mental recovery (e.g. mood and concentration) can take longer to resolve. Sydenham's chorea remains endemic in Māori, Pacific Islander, Aboriginal and Torres Strait Islander children in New Zealand and Australia. There is limited evidence to direct treatment of Sydenham's chorea, and clinical practice differs widely around the world. Dexamethasone is an oral steroid which targets the abnormal immune response and successfully treats other immune-mediated brain disorders, with good tolerability. TREAT-SC is a randomized, double-blinded, placebo-controlled trial which will investigate whether a three day course of oral dexamethasone safely and effectively treats the movement disorder and psychiatric symptoms of Sydenham's chorea. The trial will recruit 80 participants from study sites in Australia and New Zealand.