Before you give your child the MyKids Brain dietary supplement, which contains vitamin D3, the best thing to do is to have a 25-hydroxy vitamin D test done to see how much vitamin D they need, and then set the dose accordingly. According to EFSA (European Food Safety Authority), the right vitamin D supply is: for newborns and infants — supplementation from the first days of life, regardless of how they are fed — 400 IU/day; children (1–10 years old) — 600–1000 IU/day; teenagers (11–18 years old) — 800–2000 IU/day.