Nutrient-dense foods, which are often low in energy density (ED), are recommended for a healthy diet in infants and children. How ED changes during the transition from a complementary diet in infancy to a conventional diet is unknown. We aimed to describe the ED, the amount of energy (e.g., kcal) per weight (e.g., g), of food or beverage in infants and preschool-age children. It was hypothesized that ED would be higher among older children. The ED of food (ED-Food Only) and of food and all beverages excluding human milk and infant formula (ED-Food and Beverages) of children's (6 mo–5 y) diets were examined overall and by age subgroups using data from the NHANES (2009–2018). Survey-adjusted linear regression followed by pairwise comparisons were used to compare ED across age subgroups. The percentages of calories consumed from low-, medium-, and high-ED foods across age subgroups were also examined. Mean ED-Food Only was 1.21 kcal/g (95% CI: 1.13, 1.29 kcal/g) among 6- to 11-mo-olds and 1.62 kcal/g (95% CI: 1.54, 1.69 kcal/g) among 12- to 17-mo-olds (P < 0.05). ED-Food and Beverages was higher across consecutive age subgroups from 0.99 kcal/g (95% CI: 0.96, 1.02 kcal/g) in 12–17 mo through 3 y (1.22 kcal/g; 95% CI: 1.19, 1.26 kcal/g; P < 0.05). Mean percentage of calories consumed from low-ED food (≤1.0 kcal/g) became lower with age from 6- to 11-mo-olds (47.3%; 95% CI: 44.3%, 50.4%) through 18- to 23-mo-olds (16.2%; 95% CI: 14.5%, 17.9%; P < 0.05). A greater percentage of calories was consumed from high-ED food (≥3.0 kcal/g) among 18- to 23-mo-olds (39.0%; 95% CI: 37.1%, 40.9%) than among 12- to 17-mo-olds (34.0%; 95% CI: 32.0%, 35.9%; P < 0.05). ED increased across age subgroups, driven by a decrease in the percentage of calories consumed from low-ED food and an increase in the percentage of calories consumed from high-ED food.