This study aimed to validate the ability of a 24-h food diary (the DIET-24) to accurately detect change in children's fruit and vegetable consumption at school snack time following implementation of the Food Dudes healthy eating intervention. Participants were 4- to 9-year-old children from two primary schools in England. There were 148 participants in the intervention school and 43 participants in the no intervention control school. For each child, snack-time fruit and vegetable consumption was measured separately by weight (grammes), and compared with teachers' estimates (to the nearest half portion) using the DIET-24. Both consumption measures were taken at T1 (pre-intervention) and T2 (post-intervention). At each time-point, Spearman rank correlations between the two measures were low to moderate, but significant. However, when compared with weighed measures, the DIET-24 did not always accurately detect significant changes in children's fruit and vegetable consumption following the intervention. To provide sensitive measures of behaviour change, it is important that dietary measures assess as accurately as possible the amount of food consumed, rather than, as is often the case, rely on all-or-none portion estimates. This issue is important for the establishment of a reliable evidence-base for healthy eating interventions.