question: A novel coronavirus, SARS-CoV-2, has spread widely throughout the world. To reduce the spread of infection, children are prevented from going to school and have fewer opportunities for in-person communication. Although the pandemic has impacted the everyday lives of children, its impact on their development is unknown. This cross-sectional study compared Japanese children’s socio-emotional behaviors before and during the pandemic. Parents completed a web-based questionnaire before and during the pandemic for children ages 4-9. Children’s socio-emotional development in an everyday context was assessed using the Strengths and Difficulties Questionnaire (SDQ). The results indicated that during the pandemic, children were more prosocial and experienced more problems in their peer relationships, but no differences were found in emotional symptoms, conduct problems, hyperactivity between before and during the pandemic. Overall, our results suggest the pandemic may have immediate impact on children’s socio-emotional behaviors. We discussed the results in terms of behavioral immune system.