Validity and Reliability of a Food Frequency Questionnaire (NFFQ-Kids) to Assess Food Consumption Based on the Nova Classification in Southern Italian Children and Adolescents

Background/Objectives: There is a lack of food frequency questionnaires (FFQs) specifically designed to assess food consumption based on processing levels in younger populations. This study evaluates the validity and the reliability of a 107-item FFQ (NFFQ-Kids) in estimating the intake (g/day) and the weight and energy ratios of Nova groups in Italian children and adolescents aged 2–18 years. Methods: The NFFQ-Kids was administered twice (T0 and T1), with a four-week interval. A total of 73 participants completed the questionnaire at T0, and 53 completed it at T1. Participants were selected from the ICARO Study (December 2023–April 2024), a web-based cohort study in Southern Italy. Questionnaire validity was assessed by comparison with a 3-day food record (two weekdays and one weekend day) collected between the two NFFQ-Kids administrations. Test–retest reliability was evaluated to assess the consistency of the questionnaire over time. Results: A moderate correlation was found between the NFFQ-Kids and the 3-day food record for the energy ratio of ultra-processed foods (UPFs) (r = 0.48; p < 0.001; ICC = 0.46; 95% CI 0.29–0.64) and the weight ratio for both unprocessed or minimally processed foods (r = 0.49; p < 0.001; ICC = 0.50; 95%CI 0.33–0.66) and ultra-processed foods (UPFs) (r = 0.58; p < 0.001; ICC = 0.58; 95%CI 0.42–0.72). Overall, the NFFQ-Kids showed good test–retest reliability across all Nova group intakes, total food, and relative energy and weight ratios of UPFs (r = 0.71, ICC = 0.67; and r = 0.79, ICC = 0.67, respectively), indicating stable measurements over time. Conclusions: The NFFQ-Kids demonstrated acceptable validity and good reliability, proving useful for assessing food intake by processing level in Italian youth.