This clinical study is for kids aged 2-17 with ulcerative colitis (UC), a condition where the colon gets inflamed. The study tests a medicine called **tofacitinib** to see if it helps and is safe. All kids will start with a tofacitinib dose similar to what adults take, with a chance to increase the dose if needed. The study lasts up to 180 weeks, which is about 3 and a half years, and involves several phases, including an initial period to see how participants react to the medicine and a longer phase to maintain the treatment. Participants will have a follow-up visit 4 weeks after their last dose and a phone call 8 weeks later to check for any side effects. The main goal is to see if kids get better by measuring their symptoms after 44 weeks. Eligible kids must have a confirmed UC diagnosis and meet other health criteria.

• Study duration is up to 180 weeks.
• Initial open-label treatment with possible dose increase.
• Includes multiple phases with follow-up visits and phone check-ins.