Shifts toward a higher-fat, lower-fiber diet, physical activity reductions, and consequent rises in overweight are occurring in newly industrialized and developing countries experiencing rapid socioeconomic development. Existing research on these dietary and activity changes in newly industrialized countries has primarily focused on adults, and less is known about longitudinal diet and activity behaviors in children. Integration of adult and child diet and activity research is also important given significant parent-child relationships for diet and activity behaviors reported in developed countries. Furthermore, comparison of the relative contribution of diet and activity behaviors toward increased body mass index in adults versus children is needed given differences in the onset and progression of the overweight epidemic in the two population subgroups. Using longitudinal data from the China Health and Nutrition Survey, we compared dietary and PA behaviors over time and examined how these behaviors relate to increased BMI in mothers versus children. Mother-child associations for dietary, PA, and BMI patterns were also studied. We documented shifts toward diets higher in animal-source foods and lower in grains (i.e., less traditional Chinese diets) in mothers and children, with mothers versus children experiencing more pronounced shifts toward a less traditional Chinese diet over time. However, dietary correlates of increased BMI in mothers and children were less conclusive. PA trends in mothers versus children were disparate. Children reported increases in commuting and leisure-time sports activities over time, whereas mothers reported declines in commuting activity and minimal changes in leisure-time sports activity. A large proportion of mothers and children engaged in high sedentary behavior; high sedentary behavior was associated with increased BMI in mothers. We also found significant mother-child associations for dietary, PA, and BMI patterns that were consistent with those documented in developed countries. Our research provides an initial look at dietary, PA, and BMI dynamics in mothers versus children within a rapidly changing nutrition and PA environment. Our findings also support dietary interventions and PA promotion focused at the family-level for improved effectiveness and broader public health impact.