All children, regardless of age or weight, should get at least 400IU (International Units) of vitamin D a day. They should get this as a supplement if they aren't drinking at least 1000 mL (about 33 ounces) of vitamin-D fortified formula or milk a day.
Most children aren't drinking that much milk. Many of them shouldn't be. For some children, drinking a lot of milk can lead to anemia or constipation. And some children who drink a lot of milk fill up on it and then don't eat other foods like fruits, vegetables, and meat.
Vitamin D is the "sunshine vitamin," because our body makes it naturally when exposed to sunshine. The problem is that not everyone is in the sun long enough to make enough vitamin D. And it's not easy to know how long you need to be in the sun. It depends on how dark you skin is (darker skin blocks the sun more) and on the latitude where you live. The further from the equator you live, the less strong the sun.
Because of this, I've been putting my patients on a multivitamin that contains at least 400IU of vitamin D. (Most have this much, but read the label to be sure.)
Check with your doctor. Some children may need more than that. Together with your doctor, you can decide what's best for your child.