They will ingest the small gravel on the ground, the grit you have is fine, you can smash it up if you like, but they will do ok with what they find actually.
If you got them from a hatchery, they were vaccinated, and should be fine in this area also.
They are plenty old enough to forage and they will love getting their feet dirty and finding some soft ground to take a dust bath etc...
keep taking pics and updating, we all love to see our girls enjoying life!
** try putting some of their food on the ground so they can scratch around and find it...they will love it!