The gut microbiome plays a fundamental role in child development, influencing metabolic regulation, immune maturation, and neurodevelopment from early life onward. Increasing evidence links disturbances in gut microbial composition to pediatric disorders, including obesity, allergic diseases, autoimmune conditions, neurodevelopmental disorders, and gastrointestinal pathologies. This review summarizes current knowledge on the developmental trajectory of the pediatric gut microbiome, the major environmental and biological factors shaping its composition, and key disease-related microbial patterns described in recent literature. Particular attention is given to mechanisms involving immune modulation, intestinal barrier function, microbial metabolites, and the microbiota–gut–brain axis. Finally, the potential role of probiotics as a microbiome-targeted strategy is discussed, emphasizing both their therapeutic promise and existing limitations. Overall, the pediatric gut microbiome emerges as a modifiable determinant of health, warranting further longitudinal and interventional research.