my older children do not drink any milk. How can they get extra vitamin d?

Vit D rich foods. Vitamin d rich foods are: liver, fatty ocean fish, egg yolks, butter, and menhaden oil. Sunlight exposure, a few hours per week, will also add up.
Sun & foods. The flesh of fatty fish (such as salmon, tuna, and mackerel) and fish liver oils are among the best sources. Small amounts of vitamin d are found in beef liver, cheese, and egg yolks. Some mushrooms provide vitamin d2 in variable amounts. Ready-to-eat breakfast cereals often contain added vitamin d, as do some brands of orange juice, yogurt, margarine and other food products.