Since my covered run has a sand floor, I sifted about 2 quarts of it into a cardboard can box, and put it in the pen with my 2 week old BJG chicks. I sprinkle a little scratch in the box every day and they love to scratch around pecking for it. The older girls that live out there eat exclusively in the sandy run so I don't have to worry about providing supplemental grit.
That gives all of them a constant source of grit, and that means I can offer them a variety of tasty treats.
I started all of them off in the brooder with about 1/2 teaspoon of coarse corn grits (polenta) mixed with a little sand. Even at just a couple of days old, they were happy to scratch and peck at the mix and they had no problems at all eating it. And because they had grit in their little gizzards, I was able to start feeding them little mealworms at 2 weeks. Now at 4 weeks, they come right to me when I walk up to the pen, and are beginning to get used to being handled because I make them take mealworms out of my hand.
Plain old builder's sand, the kind you buy in a tube at Home Depot is perfect. The pieces of grit vary in size so the little chicks can find pieces that work for them, while the older birds find the bigger chunks and gobble them.