As part of justifiable nutrition promotion, this study aimed to determine internal consistency of a dietary fat screener and to compare self-assessment to maternal assessment of fat intake of grade six (about 12 years old) learners in a South African public primary school. The children completed in school a pictorial, quantitative food frequency-type screener consisting of 10 high-fat food categories; mothers individually completed a text version. Internal consistency was measured with item-total correlations, Cronbach's alpha and the split-half method. Child–mother comparison was based on kappa (κ) statistics, McNemar's tests, Wilcoxon signed-rank test and the Bland–Altman method. In total, 101 (93.5%) children and 78 (72.2%) mothers responded. The screener was internally consistent, regardless of data source and statistical technique. For portion sizes and frequency of intake, children consistently reported higher intake than mothers. This resulted in systematic error, also evidenced by a significant difference from zero for the difference between child's and mother's final test scores for the whole group, and for boys and girls separately (always P < 0.001). In 76% of the pairs, classification into high fat or prudent intake was identical, yet the chance-corrected agreement was poor (κ = 0.16) and non-agreement was non-symmetrical (P = 0.001). Children and mothers reported high fat intakes (93% and 75%, respectively). It was concluded that the dietary fat screener was internally consistent, yet children and mothers did not agree in their assessment. The high fat intakes reported by children and mothers warrant measurement refinement and implementation of primary prevention programmes.