**Study Overview:** This study is for children aged 6 to just under 18 who have a type of illness called ANCA-associated vasculitis (AAV). AAV can cause inflammation in blood vessels and often affects organs like the lungs and kidneys. The study will test a new medicine called avacopan, along with other treatments (rituximab or cyclophosphamide), to see how well it works. Avacopan aims to help reduce the inflammation caused by AAV. **Key Points:** - **Study Duration:** Participants will be involved for several weeks, depending on their response to treatment. - **Visits and Tests:** Regular visits to the study clinic for check-ups and tests will be required. - **Eligibility:** Must be diagnosed with granulomatosis with polyangiitis (GPA) or microscopic polyangiitis (MPA) and meet other specific health criteria. Participants should not have other autoimmune diseases or require certain ongoing medications that could interfere with the study results. This is important for ensuring the safety and accuracy of the study findings.