While you will be told that chickens do not need grit if all they are eating is processed chicken feed, that is only one opinion. Birds were created with a gizzard. That gizzard is designed to hold grit which pulverizes all of the food that they eat. Your chicks are eating the shavings because they have an instinctive drive to put something in their gizzards. Studies show that chickens that have access to the right size grit starting soon after hatch actually have better feed conversion rates, grow better, and produce more eggs.
A plug of sod will give them: first grit, minerals, beneficial bacteria and fungi to populate their guts to build a healthy immune and digestive system. First greens, some seeds, tiny insects and perhaps a worm or two, opportunity to practicing scratching in the soil like the big chickens do! First dust bath, and lots of play opportunity. Last, but IMO of great importance, that sod will give them exposure to the cocci living in your soil. They need to be exposed to the cocci, and build their immunity. Their natural immune response is greatest within the first 2 weeks of hatch while they have immunity received from their mothers.