The aim of this study was to (a) compare children’s perspectives of problematic eating behaviours with those of mothers and (b) check if there are differences in the level of these problematic eating behaviours between girls and boys in different age groups (young children: 8–11 years old vs. adolescents: 12–16 years old). The study involved 203 children (50.74% girls) and 203 mothers. The average age of children was 11.06 years (SD = 2.31), and the average BMI was 18.27 kg/m2 (SD = 2.29). Two questionnaires were used to assess children’s perspectives of problematic eating behaviours: The Three-Factor Eating Questionnaire (TFEQ-R13) and the Dutch Eating Behaviour Questionnaire for Children (DEBQ-C). One questionnaire was used to evaluate mothers’ perspectives: The Child Eating Behaviour Questionnaire (CEBQ). The main results in the study indicate the following: (a) the children’s perspective based on the DEBQ-C is the most effective at predicting their BMI (this model of problematic eating behaviours explains 29% of the variance in the child’s BMI); and (b) for almost all problematic eating behaviours, older girls have the highest levels. From the current study, it can be concluded that the type of questionnaire (TFEQ-R13 vs. DEBQ-C vs. CEBQ) and the perspective (child vs. mother) differentiate the results obtained regarding the assessment of children’s problematic eating behaviours and their relation to BMI.