Children with cancer who require pediatric intensive care encounter overlapping metabolic insults from cancer biology, anticancer treatment, and critical illness. These insults may produce a dynamic malnutrition phenotype characterized by impaired intake or absorption, rapid lean-mass loss, micronutrient and electrolyte depletion, and assessment confounded by inflammation and fluid shifts. In this narrative review, we conducted a systematic search on PubMed/MEDLINE through August 11, 2026 and prioritize evidence in the following order: pediatric oncology pediatric intensive care unit (PICU), general PICU, non-critical pediatric oncology or hematopoietic stem-cell transplantation, adult clinical, and preclinical studies. We propose an unvalidated triple-hit model and an evidence-informed decision framework. Serial measured weight should be interpreted with cumulative fluid balance and edema; bioelectrical impedance phase angle and the C-reactive protein-to-albumin ratio are prognostic or risk markers, while muscle ultrasound documents tissue trajectory but does not determine protein requirements. Enteral nutrition is preferred after adequate resuscitation when the gastrointestinal tract is usable. General PICU evidence, including the PEPaNIC trial, supports withholding routine supplemental parenteral nutrition during the first seven days, although applicability to pediatric oncology PICU populations remains unexplored. Universal energy and high-protein targets are not supported; prescriptions should reflect age, illness phase, measured energy expenditure when available, organ function, and refeeding risk. Direct evidence remains sparse, and the proposed framework requires prospective validation.