The aim of this study was to investigate changes in the eating behaviour of children and adolescents during the first lockdown implemented in Greece due to COVID-19 and to explore possible correlations with corresponding changes in the eating behaviour of their parents. A quantitative cross-sectional study was performed using an online questionnaire. The research sample consisted of 397 parents with children aged 2-18 years, who were recruited from 63 municipalities in Greece. It was observed that during the lockdown period, children/adolescents and parents reduced their fast-food consumption (the percent of parents reporting that “Never” consume fast-food increased by 45.1% and of children/adolescents by 34%). Furthermore, 85.1% of the parents reported increase in their snack consumption and 60% in their sweets consumption. An increase in consumption of sweets by children/adolescents was also observed (2.3% daily, 8.8% weekly). In addition, children/adolescents and parents increased their breakfast consumption by 5.1% and 10.6%, respectively. The increase observed in parental breakfast consumption was associated with increase in their children’s/adolescents’ breakfast consumption (46.9%), while the decrease in parental breakfast consumption was associated with decrease in their children’s/adolescents’ breakfast consumption (60%) (p=0.01). In conclusion, negative changes were found in the eating behavior of children/adolescents and their parents during the first COVID-19 lockdown in Greece. Considering that lockdowns may be applied again to tackle the COVID-19 pandemic, the findings of the present study indicate that the promotion of healthy nutrition should be included in the agenda of public health measures to be taken.