Abstract Introduction: Intensive chemotherapy protocols and hematopoietic stem cell transplantation (HSCT) in children with cancer frequently lead to severe complications, such as mucositis and immune dysfunction. A growing body of evidence indicates that these complications are closely associated with the patient’s nutritional status and the composition of the gut microbiome, which becomes profoundly destabilized as a result of cytotoxic therapy and antibiotic use. Background: The aim of this review is to critically evaluate the current state of knowledge on the interplay between gut dysbiosis, metabolomic profiles—with particular emphasis on short-chain fatty acids (SCFAs)—and treatment-related toxicity in pediatric patients, as well as to delineate pathways toward personalized nutritional therapy. Methods: A narrative review was conducted, including clinical and preclinical studies published between January 2015 and October 2025. PubMed/MEDLINE, Embase, Cochrane Library, and other databases were searched, focusing on changes in microbiome composition, correlations between gut-derived metabolites and the severity of complications (sepsis, graft-versus-host disease [GvHD], mucositis), and the effects of targeted nutritional interventions (probiotics, prebiotics, postbiotics, and fecal microbiota transplantation [FMT]) on microbiome modulation during anticancer therapy. Results: The analysis demonstrates that pediatric oncologic treatment leads to a marked reduction in microbial diversity, including the loss of protective Clostridiales taxa (e.g., Faecalibacterium), accompanied by an overgrowth of Proteobacteria pathobionts. Metabolomic profiling indicates that low SCFA levels (e.g., butyrate < 20–50 µmol/g) are a strong predictor of severe mucositis, prolonged neutropenia, and an increased risk of sepsis. Interventions aimed at restoring eubiosis and enhancing SCFA production show potential in strengthening the intestinal barrier, modulating immune responses, and enabling maintenance of the planned relative dose intensity (RDI) of chemotherapy by reducing treatment-related toxicity. Conclusions: Gut microbiome profiling and fecal metabolomics represent promising prognostic tools in pediatric oncology. There is an urgent need for further research employing “omics”-based approaches to develop precise, individually tailored nutritional protocols. Such strategies, including postbiotics and FMT, may minimize treatment-related adverse effects and improve long-term clinical outcomes in pediatric patients. Keywords: dietary interventions; gut microbiome-targeted therapies; nutrition supplements; pediatric oncology