Selenium is an essential trace element integral to various physiological processes in children, primarily through its incorporation into selenoproteins such as glutathione peroxidase, iodothyronine deiodinase, thioredoxin reductase, and selenoprotein P. These proteins play pivotal roles in antioxidant defense, thyroid hormone metabolism, immune response, and reproductive health. Dietary selenium exists in organic forms, such as selenomethionine and selenocysteine, which is found in plant and animal products and inorganic forms such as selenite and selenate. Rich dietary sources include Brazil nuts, cereals, seafood, and dairy products. Recommended daily allowances (RDA) vary by age: infants require 20 μg/day; children aged 1–3 years, 20 μg/day; 4–8 years, 30 μg/day; 9–13 years, 40 μg/day; and adolescents 14–18 years, 55 μg/day. Absorption occurs mainly in the duodenum, with selenium transported to the liver bound to lipoproteins, where it is incorporated into selenoproteins. Deficiency can lead to conditions such as Keshan disease, an endemic cardiomyopathy prevalent in selenium-deficient regions of China, characterized by heart failure and pulmonary edema. Conversely, excessive intake can result in selenosis, manifesting as hair and nail loss, mood changes, and a garlic odor on the breath. Given the narrow margin between dietary requirements and toxic levels, strict regulation of selenium intake is crucial to prevent deficiency and toxicity.