Dietary factors play a major role in the development of non-communicable diseases, however little is known regarding the impact of nutrition on rare diseases like sarcomas. This Rizzoli Orthopedic Institute study aimed to evaluate the relative validity of a Food Frequency Questionnaire (FFQ) to measure the consumption of foods in comparison with a 3-days diary diet in a healthy Italian student population aged between 12 and 17 years. An extended version (including food groups for children) of the semi-quantitative FFQ used in the European Prospective Investigation into Cancer and Nutrition (EPIC) was administered. The validity of the FFQ was assessed by comparing the intakes from the FFQ against the 3-day diary method. 254 Italian subjects were included in the analyses: 128 females; 126 males; 116 from High Secondary School (14–17 years); 138 from Low Secondary School (12–13 years). Mean and median intakes are overall higher in the FFQs than in the food diaries. Spearman correlations adjusted for within-person variability were highest for legumes, vegetables and coffee/tea (>0.5), followed by potatoes, meat, fruits, breakfast cereals, biscuits and candies, and milk/yoghurts (>0.4). Moderate correlations were found for alcoholic drinks, soft drinks, juices, and grains (>0.3). For some food groups, such as fish, potatoes, and bread, correlations tend to become higher when stratifying the analyses for age group. These results demonstrate that the adapted EPIC COS FFQ validated in Italian adults is also appropriate and well understood by Italian children and adolescents.