It is recommended to give vitamin D to a child who is 18 months old during the winter season, as the recommended daily intake for children aged 1 to 3 years is 600 units. This amount can easily be obtained through foods such as cow's milk, soy milk, fortified orange juice, egg yolks, beef, pork, and fish. Playing outside also helps to generate vitamin D, although the amount is lower in the winter compared to summer. Unless your child does not consume milk, it may not be necessary to give them a specific vitamin D supplement.