The Collaborative Improvement and Innovation Network to Advance Care for Children with Medical Complexity (CMC CoIIN) was designed with a foundational commitment to partnership with family leaders to codesign and improve systems of care and supports for CMC and their families - this fundamental commitment was essential to the CMC CoIIN's measurement strategy. In this paper, we examine key learnings from partnering with family leaders from interdisciplinary state teams in the CMC CoIIN to identify and define quantitative quality improvement measures to improve care and support for CMC and their families, including quality of life, well-being, and flourishing; unmet health needs; and support systems such as medical home, patient and family engagement, and shared plans of care. Codesigning the CMC CoIIN measurement strategy with family leaders greatly enhanced our measurement approach and provided numerous unique learning opportunities for the CMC CoIIN's project team and state teams.