They can have finely ground scratch as long as they've had access to grit.
But why not take this opportunity to train them to come to an audio cue? That's really what people are doing when they shake a can of scratch. The chickens have been conditioned to come at the sound of scratch rattling.
Right now, today, I've begun to train my four-week olds. I use a clicker you can get at any pet store for around a dollar. I click it then show them some meal worms. I click it while feeding out those worms. After a few days of clicking while distributing meal worms (or scratch), they will associate the sound with getting their favorite treat.
My adult flock is trained to come immediately when they hear the clicker. One time, I saw a hawk fly over. My chickens were all on the opposite side of the yard, and I wanted to get them all back into the covered run as soon as possible. When they heard the clicker, all of them came running. They were back under cover and safety within fifteen seconds. They got some scratch to reinforce the training.
