The concept of Culturally and Linguistically Diverse (CALD) populations is unique to Australia. It was introduced in 1996 and is intended to refer to ethno-cultural groups that are neither Aboriginal or Torres Strait Islander nor considered from mainstream English-speaking Anglo-Celtic backgrounds. CALD children have been identified as a priority population by the Australian government because they may experience inequities in health outcomes compared to Anglo-Australian children. Inequities in the health and wellbeing of CALD children are driven by myriad processes including racial discrimination, socioeconomic disadvantage, and limited access to health services. But who are CALD children? Despite the availability of statistical standards for data collection on CALD characteristics such as country of birth and language spoken, the concept itself lacks an official operational definition. Applying definitions specified by various organisations to data from the 2016 Australian Census, the estimated proportion of CALD children ranged from 11% to 44% of Australian children aged 0 to 17 years. There are few published studies on CALD children in Australian child health research, with most studies focused on refugees. There is no consensus on how CALD is defined in child health research in Australia. We propose several considerations in the use of the CALD concept in child health research. This includes adhering to the Australian Bureau of Statistics standards on Cultural and Linguistic Diversity, use of multiple indicators to identify CALD, and acknowledging the significant heterogeneity of CALD communities which may contribute to observed differences in health. If we are to advance health and well-being equity for CALD children, we need a more carefully considered and consistent approach to understanding which children are CALD.