Balanced nutrition is crucial for adolescent's proper physical and mental development. Dietary habits change significantly with a child's development. Along with increasing age and the shift towards adolescence, unhealthy diet-related habits become more common. The objective of the survey study was to determine the differences in nutritional habits between children and adolescents according to their age and body mass index (BMI). "Let's get the kids moving" campaign (pol. "Uruchamiamy dzieciaki") was launched in 2016. Within the campaign, the survey study was conducted in 2913 participants between 6 and 17 years old from primary and junior high schools in Wroclaw (Poland). The survey was anonymous, and its supplement was voluntary. Participants were divided into age groups. The study group of 2913 consisted of 29.8% of 6-9-year-olds, 32.7% of 10-12-year-olds, and 37.5% of 13-17-year-olds. Body mass index (BMI) was calculated and further interpreted as a BMI z-scores depending on children's age and gender. A total of 19.3% of participants consumed 3 meals a day or less. Children from the oldest age group (13-17) consumed statistically significantly fewer meals per day than younger children (p < 0.001). Children from the oldest age group (13-17) consumed breakfast statistically less often than children of age group 10-12 years (75.0% vs. 83.6%; p < 0.001) and children of age group 6-9 years (75.0% vs. 84.0%; p < 0.001). Severely thin children consumed breakfast significantly more often than overweight (85.8% vs. 76.3%; p = 0.004) and children with obesity (85.8% vs. 75.9%; p = 0.021). Children with obesity consumed vegetables significantly less often than severely thin (p < 0.008), thin (p < 0.001), and children with normal body weight (p < 0.007). The oldest children (13-17 years) consumed Coca-Cola and SSB (p < 0.001) and fruit-flavored beverages (p < 0.05) significantly more often than children from other age groups. Boys consumed carbonated beverages with added sugar significantly more often than girls (p < 0.01). Unhealthy diet-related behaviors in children and adolescents may promote overweight and obesity and should be targeted in health promotion programs. Special attention should be paid to 13-17-year-olds, as adolescents from this group made more unhealthy choices than younger children.