Studies have shown preventive effects of an active lifestyle during childhood on later life; therefore, health promotion has to start early. The programme “Join the Healthy Boat” promotes a healthy lifestyle in primary school children. In order to evaluate it, children's behaviours in respect of increased physical activity (PA), a decrease in screen media use (SMU), more regular breakfast, and a reduction of the consumption of soft drinks (SDC) were investigated. 1943 children (7.1 ± 0.6 years) participated in the cluster-randomised study and were assessed at baseline and 1736 of them at follow-up. Teachers delivered lessons, which included behavioural contracting and budgeting of SMU and SDC. Daily SMU, PA behaviours, SDC, and breakfast patterns were assessed via parental questionnaire. After one-year intervention, significant effects were found in the intervention group for SMU of girls, children without migration background, and children with parents having a low education level. In the control group, second grade children skipped breakfast significantly more often. Tendencies but no significant differences were found for PA and SDC. This intervention seems to affect groups, which are usually hard to reach, such as children of parents with low education levels, which shows that active parental involvement is vital for successful interventions.