Background Dengue fever is a mosquito-borne viral disease that is associated with four serotypes of the dengue virus. Children are vulnerable to infection with the dengue virus and the risk of severe dengue disease is the highest among infants and children, particularly those who have been previously infected with a different dengue serotype. Sufficient knowledge, positive attitudes, and proper practices (KAP) are essential for dengue prevention and control. This study aims to estimate the dengue seropositivity for study participants and to examine the association between households’ dengue-related knowledge, attitudes, and practices (KAP), and children’s risk of dengue seropositivity, while accounting for socioeconomic and demographic differences in Brazil. Methodology/Principal Findings This analysis was based on a cross-sectional study from Fortaleza, Brazil between November 2019, and February 2020. There were 392 households and 483 participant children who provided a sample of sufficient quality for serological analysis. The main exposure was a household’s dengue-related knowledge, attitudes, and practices, assessed through a questionnaire to construct a composite KAP score categorized into three levels: low, moderate and high. The main outcome is dengue immunoglobulin G(IgG) antibodies, collected using dried blood spots and assessed with Panbio Dengue IgG indirect ELISA (enzyme-linked immunosorbent assays) test commercial kits. The estimated crude dengue seroprevalence among participating children (n=483) was 25.1%. 5% (n=20) of households achieved a score of over 75% for KAP, while most households achieved between 50% and 75% of the total scores (n=271, 69%), and 101 households scored lower than 50% of the total score (26%). Each KAP domain was significantly and positively associated with the others. We found high household KAP scores were associated with an increased risk of seropositivity (RR: 2.08, 95% CI: 1.09-3.97, p=0.027). Household adult respondents’ education level of elementary school or higher was negatively associated with children’s relative risk of being seropositive (RR: 0.65, 95% CI: 0.49-0.87, p=0.004). The risk of seropositivity in older children (6-12 years old) was over 6 times that of younger children (2-5 years old) (RR: 6.15, 95% CI: 3.51-10.76, p<0.001). Children living in households with sealed water tanks or no water storage had a lower relative risk of being seropositive (RR: 0.73, 95% CI: 0.55-0.99, p=0.041). Conclusions/Significance Our results provide insight into the prevalence of dengue seropositivity in Fortaleza, Brazil in children, and certain demographic and socioeconomic characteristics associated with children’s risk of being seropositive. They also suggest that KAP may not identify those more at risk for dengue, although understanding and enhancing households’ KAP is crucial for effective community dengue control and prevention initiatives.