Purpose <p>Children with solid tumors are highly vulnerable to treatment-related nutritional compromise. However, longitudinal data describing how dietary intake and nutritional risk evolve during therapy remain limited. This study aimed to examine longitudinal changes in dietary intake adequacy and nutritional risk across different phases of oncological treatment and to identify clinically vulnerable periods requiring intensified supportive care.</p> Methods <p>In this prospective longitudinal study, children aged 8–18&#xa0;years with newly diagnosed solid tumors were assessed at three predefined treatment phases (early-, mid-, and late-treatment). Dietary intake was evaluated using interviewer-verified 3-day food records at each time point. Energy and nutrient intakes were expressed as percentages of age- and sex-specific estimated requirements. Nutritional risk was assessed using the Screening Tool for Childhood Cancer Nutrition Risk (SCAN). Longitudinal changes were analyzed using non-parametric repeated-measures methods, and associations with gastrointestinal symptoms and taste alterations were explored.</p> Results <p>Sixty children completed all assessments. Energy adequacy was below recommended levels at all assessment points and declined significantly over time (<i>p</i> = 0.001), with most patients failing to meet recommended energy requirements throughout treatment. Significant reductions were observed in carbohydrate, fat, dietary fiber, and selected micronutrients, particularly iron and magnesium. Absolute protein intake remained statistically unchanged over time, whereas the relative contribution of protein to total energy intake increased significantly. More than 80% of patients were classified as being at nutritional risk at all time points. Higher nutritional risk scores were significantly associated with greater gastrointestinal symptom burden and taste alterations (all <i>p</i> &lt; 0.05). Nutritional vulnerability was present across all assessment points. The mid-treatment phase was characterized by relatively greater deterioration in dietary adequacy and nutritional risk indicators; however, inadequate intake and elevated nutritional risk persisted throughout treatment.</p> Conclusions <p>Children with solid tumors experience progressive deterioration in dietary intake adequacy during oncological treatment, with nutritional vulnerability evident across the treatment trajectory and relatively greater deterioration observed during mid-treatment. Future supportive care strategies should move beyond monitoring intake alone and focus on identifying modifiable factors driving dietary decline, including gastrointestinal symptoms and taste alterations, to determine the extent to which this deterioration can be prevented or reversed and when nutritional interventions may be most effective.</p>