I took Holderread's lead on this, adding just a sprinkle (teaspoon or two) of chick grit on their morning bowl (6" diameter) of crumbles. This was for ducklings. A few months later when they were outside most, then all the time, I stopped, since they get grit from foraging.
I would add it for ducklings into their food, as there was a story in Storey's Guide to Raising Ducks about a duckling that perished because it filled up on the free choice grit. This is likely very rare, but I did not want to risk it.