The social isolation enforced as a result of the new coronavirus (COVID-19) pandemic may impact families’ lifestyle and eating habits. The present study aimed to assess the behaviour and dietary patterns of Brazilian children and adolescents during the social isolation imposed by the COVID-19 pandemic.The present study was conducted using an online, anonymous cross-sectional survey with 589 children and 720 adolescents from Brazil during a nationwide social isolation policy. The Mann-Whitney U-test or the Kruskal-Wallis with the Dunn post-hoc method and a radar chart were used to compare the weekly consumption of each food by age group and isolation status. p < 0.05 was considered statistically significant. Analyses were conducted using R statistical software, version 4.0.2 (R Foundation for Statistical Computing). We found that isolated families showed breakfast eating habits and the consumption of raw salad, vegetables, beans and soft drinks. Lower-class isolated families and those from the Northeast region consumed fruits, juices, vegetables and beans less frequently. Compared to children, adolescents were less isolated (p = 0.016), less active (p < 0.001), exposed to longer screen time (p < 0.001), showed an inadequate sleeping pattern (p = 0.002) and were from lower-class families (p < 0.001). Social isolation affected the eating habits of children and adolescents. Non-isolated families presented a lower consumption of healthy food, especially those among the lower class, from Northeast Brazil, as well as adolescents.