All participants will be administered vedolizumab via intravenous (IV) infusion. Participants assigned to the low dose group who do not achieve clinical response (based on pediatric ulcerative colitis/Crohn's disease activity index) at Week 14 will receive the high dose (ie, 300 mg for participants ≥30 kg baseline weight and 200 mg for participants <30 kg baseline weight) of vedolizumab IV at Week 14.