The rapid changes in social and economic conditions in many populations may have significant impacts on health, including child oral health. Understanding trends and variations between and within countries will assist in the development of effective preventive programs. This review aims (1) to document time-trends in child caries experience of countries with different levels of social and economic development, and (2) to compare factors affecting the caries experience of children in two contrasting countries. A time-trend analytical approach of ecological data from countries with different levels of social and economic development and individual data of the two contrasting Vietnamese and Australian child populations were used. The analysis found a significant decline in caries experience of children, mostly driven by the improvement in populations with high social and economic development. Significant variations in caries experience and in associations with risk factors between and within countries were observed. Socio-economic inequality in child oral health existed within developed countries and between countries with different levels of development. Population programs aimed at improving upstream factors are of priority in further improving child oral health in different populations.