Childhood obesity remains one of the most serious medical challenges of the 21st century. The aim of the study was to obtain epidemiological data on the prevalence of overweight and obesity among 8- and 9-year-old children in Szczecin, and to evaluate the effectiveness of medical intervention in the form of a year of interdisciplinary work with children with excess body weight. The study consisted of two main stages: I—screening, II—intervention. The program was implemented for three consecutive years, starting in 2016–2018. The entire population of 8–9-year-olds in Szczecin is 11,494 children. In the screening part of the study, 4890 children took part, whose parent agreed to participate (42.54%). In the intervention part of the study, we analyzed a group of 515 children. Children were further divided into subgroups according to the number of visits completed. Anthropometric parameters were measured on each visit. The prevalence of overweight and obesity in the screened population was 16.9% and 6.4%, respectively. Statistically significant changes were observed in BMI (Body Mass Index) percentiles and BMI z-scores, as well as WHR (Waist-Hip Ratio) during the one year observation time. The best effects were achieved by the 3rd visit (for the first 6 months of the program). Thereafter, the effects diminished due to the longer interval between the 3rd and 4th visits (6 months). There is the need for long-term programs for the prevention of excessive body weight in children and adolescents with frequent checkpoints.