Reliable assessments of health-related behaviours are necessary for accurate evaluation on the efficiency of public health interventions. The aim of the current study was to examine the reliability of a self-administered primary caregivers questionnaire (PCQ) used in the ToyBox-intervention. The questionnaire consisted of six sections addressing sociodemographic and perinatal factors, water and beverages consumption, physical activity, snacking and sedentary behaviours. Parents/caregivers from six countries (Belgium, Bulgaria, Germany, Greece, Poland and Spain) were asked to complete the questionnaire twice within a 2-week interval. A total of 93 questionnaires were collected. Test-retest reliability was assessed using intra-class correlation coefficient (ICC). Reliability of the six questionnaire sections was assessed. A stronger agreement was observed in the questions addressing sociodemographic and perinatal factors as opposed to questions addressing behaviours. Findings showed that 92% of the ToyBox PCQ had a moderate-to-excellent test-retest reliability (defined as ICC values from 0.41 to 1) and less than 8% poor test-retest reliability (ICC<0.40). Out of the total ICC values, 67% showed good-to-excellent reliability (ICC from 0.61 to 1). We conclude that the PCQ is a reliable tool to assess sociodemographic characteristics, perinatal factors and lifestyle behaviours of pre-school children and their families participating in the ToyBox-intervention.