Step 1: Recognise the diagnosis: Sydenham chorea is the neurological component of rheumatic fever, producing involuntary dance-like movements in a child. The goal of treatment is to calm the over-active movement pathway.
Step 2: Two broad drug strategies exist - increasing inhibitory GABA tone or blocking dopamine. The safer and favoured route in resistant cases is the GABA-enhancing one, and valproate is the standard GABA-acting drug used here, with benzodiazepines as a back-up.
Step 3: Eliminate the rest. Haloperidol and risperidone are dopamine blockers that work but invite extrapyramidal toxicity, making them second-line. Warfarin thins blood and is irrelevant to movement disorders. Hence valproate is the answer.\[\boxed{\text{Valproate}}\]