I made sure to give them tiny sprinkles of chick grit in their crumbles, and started with peas when they were a week old. They were the petit peas, and not too many, but we did start early with grit, then "real food" for treats. I chopped up lettuce very finely and served it in shallow water. Heaven! A mess, but I would do it right before brooder cleanup time, so no additional work was required.
Speaking of peas, thanks for the reminder! I'm going to plant some today.