A Study to Evaluate the Pharmacokinetics, Safety and Efficacy of Afimkibart (RO7790121) in Children With Moderately to Severely Active Ulcerative Colitis

Inclusion Criteria: * Bodyweight \>= 10 kilogram (kg) * Confirmed diagnosis of UC * Moderately to severely active UC, defined as modified Mayo Score (mMS) or 5 to 9 points, including endoscopic score of 2 or 3, confirmed by centrally read endoscopy (flexible sigmoidoscopy or colonoscopy) * Demonstrated intolerance or inadequate response (IR) to one or more of the following categories of drugs: systemic corticosteroids, immunomodulators, and/or biologic therapies as outlined in the protocol Exclusion Criteria: * Monogenic disorder pertaining to infant onset inflammatory bowel disease (IBD) * Current diagnosis of Crohn's disease (CD), abdominal/intrabdominal/perianal fistula and/or abscess, indeterminant colitis, IBD-unclassified, microscopic colitis, ischemic colitis, infectious colitis, radiation colitis, or active diverticular disease * Presence of an ostomy or ileoanal pouch * Current diagnosis or suspicion of primary sclerosing cholangitis * Any major surgery within 6 weeks prior to screening or a major planned surgery during the study * Active tuberculosis (TB) infection suggested by positive TB testing, clinical symptoms, and/or chest imaging (X-ray or CT)