They probably are getting grit that way. You'd be seeing crop issues if they weren't getting any though as they can't process it without grit.
That one chick developing symptoms could be the first one and more may show up later. It's good for them all if you stop the scratch, try the wet crumbles as they'll think that's a treat, and some scrambled eggs now and then for treats. Try this as well as your vitamin supplements, and you should start seeing improvement in that one, and all of them might actually look a little better, as in feather development, etc.