There is increased social concern regarding children's weight in China, but there is a relative lack of research concerning its social determinants. Based on 1,656 school-age children's samples in mainland China from the Chinese Nutrition and Health Database (CHNS 2011), we analyzed the impact of multiple factors on children's weight using a welfare mix framework that integrated factors including family living arrangements, economic development, and educational spending. School-age children's weight was measured using body mass index (BMI). The independent variables were factors developed based on the social determinants of health perspective in conjunction with a welfare mix framework. These variables included family living arrangements to reflect the family welfare factor, average government educational expenditure per student to reflect the state welfare factor, and GDP per capita to reflect the market welfare factor. Multiple regression models were used to analyze the influences of each factor on children's weight. The results showed that the factors were significant, with different directions of influence on weight. The results also showed that the factors considered in the welfare mix framework sometimes serve as risk factors rather than solely protective factors within the Chinese context. According to this study, a reasonable geographical distribution of health service resources and child-oriented health policies are needed to promote children's well-being.