Studies on children’s nutritional behaviour (CNB) rarely compare children’s answers regarding the frequency of their sugar intake with the respective statements of their parents. Therefore, data from a prevention study were used to analyse this aspect, as well as a potential correlation between Marburg Sugar Index (MSI) values and caries experience of children. The present study based its questionnaire data on CNB and caries data. Pairs of questionnaires filled in separately by children and parents of the participating families were dichotomised by either having completed the diet section entirely (Group A) or in part (Group B). The MSI scores were calculated separately for children and parents. The statistical confidence level was set at α = 0.05 (two-sided). Furthermore, the Pearson correlation coefficient was calculated and tested for “r = 0”. Additionally, a test for equality of the correlations was applied. The number of available questionnaire pairs was 429 in Group A and 400 in Group B. In both groups, significant correlations between children’s and parents’ MSI scores (A: r = 0.301, p < 0.001; B: r = 0.226, p < 0.001) were found. Using Spearman’s Rho, a significant correlation between MSI scores and children’s caries experiences was observed in Group A. MSI scores based on dietary questionnaires can be used to obtain consistent information on children’s CNB provided by the children themselves or their parents. This is true even when the MSI score has to be calculated on the basis of incomplete questionnaires. Questionnaire-based CNB information can improve the effectiveness of individual or group preventive measures supplemented by individually adapted nutritional counselling.