Background: Assessment of diet and physical activity and their determinants still remains a demanding task, especially when the objective is to evaluate the efficacy of lifestyle interventions. In the context of the Feel4Diabetes study (a European community based intervention study in families with school aged children and at high risk of developing diabetes), we aimed to develop questionnaires for the assessment of food-frequency and eating behaviors, and physical activity and sedentary behaviors in both parents and school-aged children and a questionnaire for overall family’s energy balance-related behaviors. Methods: Questionnaires were developed to be used in 6 countries under standardized harmonization procedures and included questions regarding not only food intake and physical activity, but also questions of their determinants. A reliability study was conducted in 191 pairs of parents and their children (N = 191). Parents completed the questionnaires on two occasions, within a 1–2 week interval. Reliability was tested by the intra-class correlation coefficients (ICC) of test-retest. Results: Most of the questions in all questionnaires had excellent reliability, assessed as an ICC of > 0.810. Mean ICCs for food-frequency and eating behaviors questionnaires were 0.838 and 0.787, and for physical activity and sedentary behaviors questionnaires were 0.734 and 0.793, in adults and children respectively. Mean ICC for overall family’s energy balance-related behaviors and their determinants was 0.659. Conclusion: The developed questionnaires showed acceptable reliability and may be valuable tools in the assessment of children’s and parents’ behaviors related to diet, physical activity, sedentary behavior and overall energy balance in school- and community-based interventions.