Purpose: During the COVID-19 pandemic, closing schools was discussed to limit the virus transmission, despite a lack of evidence on the role of young children in the virus spread. We undertook a study among Belgian primary schools to assess the SARS-CoV-2 antibody seroprevalence and its relationship with local incidence, school size, and socioeconomic status (SES). Methods: Schools were purposively selected using three criteria: an area with either a low or a high COVID-19 incidence, either a small or a large size, and either a low or a high SES. 932 (/2488, 38%) children and 242 (/444, 55%) staff signed informed consent. COVID-19 antibodies were tested using rapid finger prick tests. Results: Children participation was positively correlated with staff (r = +0.33; 95% CI [−0.34; 0.78]), and strongly with SES (r = +0.81; 95% CI [0.40; 0.95]). Seroprevalence was 21% in children (191/922) and 25% in staff (61/240). Seroprevalence did not correlate with local cumulative incidence (children: r = +0.06; 95% CI [−0.59; 0.67]; staff: r = +0.26; 95% CI [−0.40; 0.74]). In staff, seroprevalence was higher in schools with higher SES (r = +0.37; 95% CI [−0.29; 0.79]), but not in children (r = −0.10; 95% CI [−0.66; 0.53]). Conclusion: The lower seroprevalence in children suggests they are lower transmitters than adults, but poor socioeconomic levels were less representative.