**Study Summary** This study is for children with *moderately to severely active ulcerative colitis (UC)*, a condition where the colon and rectum become inflamed. The goal is to test a medicine called **Afimkibart (RO7790121)** to see if it is safe and effective. This is a *Phase III study*, which means it is one of the last steps before the medicine can be approved for general use. The study is *randomized and double-blind*, meaning neither the participants nor the researchers know who is receiving the actual medicine or a placebo (a fake treatment), to ensure unbiased results. **Eligibility:** Children must weigh at least 10 kilograms and have UC. They should have tried other treatments like *systemic corticosteroids* (a type of steroid medicine), *immunomodulators* (medicines that help control the immune system), or *biologic therapies* but didn't respond well. **Exclusion Criteria:** Children with other conditions like *Crohn's disease* or who have had major surgeries recently cannot participate. **Key Points:** - Study includes multiple visits and lasts several months. - Participants must have moderate to severe UC. - There may be some risks as the medicine is still being tested. **Note:** Please discuss with your doctor to see if this study is right for you.