Vitamin D is best made by our bodies rather than supplemented with food or gummies. Fresh air is a must and exposure to sunlight is the best way to allow our bodies to manufacture its own Vitamin D. This increases our mood and guess what? Our immune system.
So get outside with your kids! An idea if your kids love to be creative is to let them make a fairy garden out of whatever recycled trash you might have around the house. My kids and our neighbor kids LOVE to do this and they start with a box (shoe box or other) for a house and then they can look around for natural items outside to adorn the house or make beds, tables, and chairs. They use acorn caps for plates and bowls and paint them in cute colors. Old magazines that are in the recycling bin are used to cut out pictures of foods and other items to make it look like a real little fairy house.
What about an outdoor scavenger hunt? Or if you have older children you can do your own geocaching to get outside for some body generating Vitamin D.
This takes a little prep but my son is very involved with his Boy Scout group and he loves to make food outside. Sure, you can do a camp fire, but you might consider using your grill. Allow your kids to come up with a recipe that can be made outside on the grill or if you are savvy and so inclined you can make a fire and food according to the Boy Scout manual.