Longitudinal research from early life provides a unique opportunity to understand the development of ethnic identity as well as the challenges for ethnic identification research. Further, in Aotearoa New Zealand, longitudinal research is critical to understand development of Māori identity and the opportunities that this provides to research and implement policies and practices that enhance wellbeing and uphold our obligations to the Treaty of Waitangi. The Growing Up in New Zealand study has measured, at multiple data collection waves, the development of ethnic identity and changes in ethnic identification patterns over time and across generations for the approximately 6800 longitudinal cohort members. Trajectories towards optimal development for children described by ethnic subgroups, and comparison across ethnic subgroups are critical research questions that have been explicit since participant recruitment. To this end, the cohort is powered to ensure robust Māori, Pacific and Asian-specific analyses and interpretation towards wellbeing. Emerging ethnic identification patterns differ according to sociodemographic features, including ethnic identification itself, as well as according to who is responding and when. This paper will describe these patterns, and focus on trajectories to child-identified ethnic identification at 8 years of age. This paper will also consider the challenges involved in the robust collection of ethnic identification data in a longitudinal study, particularly in the context of maintaining Kaitiakitanga as ethical researchers and fostering Indigenous data sovereignty.