Grit is really not a treat for them. All birds need to swallow small stones to store in their gizzard, and these help them to grind up their food. Grit is usually crushed limestone and serves this purpose for them. I've never heard of them adding calcium to grit but if the stuff you bought has calcium, then it might be advisable to wait until they are of laying age to give it to them.
If they free range they will probably find all the grit they need. And, if you keep them in a pen but feed them only commercial chicken food, the need for grit is much less also. However if you are going to offer them kitchen scraps or other treats, they need to have grit available. They will only eat it when they need to though - they won't treat it as a treat.
