Nutritional quality of children’s diets is a public health priority in the fight against childhood obesity and chronic diseases. The main purpose of this study was to determine contribution of snacks to energy and nutrient intakes and to identify leading snack food sources of energy, total fat, and added sugars amongst young children in the United States. Using the 2005–2012 NHANES data, dietary intakes of 2- to 5-year-old children were analysed from a parent-reported 24-hour dietary recall (n = 3,429). Snacking occasions were aggregated to determine the proportion of total food/beverage intake obtained from snacks, estimate energy, and nutrient intakes, and identify the leading snack food sources of energy, added sugars, and total fat. Nearly all children consumed a snack on the reported day (62% morning, 84% afternoon, and 72% evening). Snacks accounted for 28% of total energy intake, 32% of carbohydrates, 39% of added sugars, and 26% of total fat and dietary fiber intakes for the day. Snacking occasions accounted for 46.6% of all beverages consumed on the reported day. Snacks and sweets food category (i.e., cookies and pastries) were the leading sources of energy (44%), total fat (52%), and added sugars (53%) consumed during snacking occasions. Sweetened beverages (e.g., fruit and sport drinks) contributed 1-quarter of all added sugars obtained from snacks. Snacks contribute considerable amount of energy and nutrients to young children’s diets, with a heavy reliance on energy-dense foods and beverages. Targeted interventions are needed to improve the nutritional quality of snacks consumed by young children.