Utah State University Extension
This fact sheet explains what vitamin D does, why a child might need more vitamin D, and three ways to increase levels of vitamin D in the body: through diet, sunlight, and supplementation.
Kunzler, Alicia; Savoie-Roskos, Mateja R.; and Durward, Carrie, "Ensuring Adequate Vitamin D Intake Among Infants and Children" (2018). All Current Publications. Paper 1905.