Their starter may have grit in it already, otherwise cleaned sand is good to use, sprinkled on food.
There's a lot you can feed for treats- mashed egg, ground oatmeal (non instant, get the good stuff and grind it with your fingers, it should pretty much fall apart), yogurt, bits of fruit (mine like strawberries and were afraid of blueberries), crushed cheerios or flakes, basically stuff little little kids would eat if you fed them healthy things.
I don't have any chicken babies right now, but my two peachicks love the wet mash I made with ground egg, ground oatmeal, plain yogurt, chick starter, and electrolyte water. It's not MEANT for them, it's MEANT for the dwarfed duckling they are in with, but they will fight me for it every time and play keep away if one of them nabs a somewhat cohesive chunk. They're very silly.