Despite the growing prevalence of obesity among children, limited research has examined the psychological and social factors that contribute to unhealthy food behaviors in developing-country contexts. This study examines how materialism, a factor contributing to obesity, impacts children's unhealthy food behaviors in Iran, a developing country. This research utilizes structural equation modeling based on data from 293 children and their parents to test the research model. Data collection involved measurements adapted from prior studies. A full-factorial, randomized, mixed-effects experimental design was then applied to test the moderating effect of health knowledge on children's food choices. Two elementary schools, randomly selected by the Department of Education, participated in this study. Of the 293 participants, 74% were girls. Statistical analysis indicated that parents' materialism significantly influenced children's materialism, which indirectly affected children's unhealthy food choices through susceptibility to peer influence. Additionally, activating health knowledge moderated the impact of materialism on children's unhealthy food behaviors. As the results supported the role of materialism in children’s unhealthy food behavior, it is crucial to find ways to control materialism in both children and parents. To reach this aim, some suggestions have been presented. The findings provide practical implications for parents, educators, and policymakers.