The COVID-19 pandemic has caused huge changes in people’s lifestyle, health, and social relationships. This situation has had an impact on children and adolescents, affecting their health, intellectual, physical, and emotional development. The survey aimed to compare eating behaviors, level of physical activity (PA), hours of sleep, and screen time among Polish children and adolescents aged 6–15 years before and during the COVID-19 pandemic. We obtained self-reported data from 1016 participants at two measurement points before and during the COVID-19 lockdown in Poland to examine the influence of the lockdown and the distance learning on PA, dietary habits, sleep, and media usage of children and adolescents aged 6–15 years. The study identified dietary differences and changes in daily activity patterns (reduced sleep duration with higher sleep quality and reduced physical activity). Additionally, the increase in general media usage was observed during the pandemic alongside a reduction in smartphone usage. Together, the findings indicate increased sleep, physical activity, and reduced media usage and screen time among Polish children and adolescents during the COVID-19 pandemic.