What is your "dirt" recipe. The only reason why I'm asking is most of my dirt has a lot of clay in it. *sigh* Planting a tree is a MAJOR excavation, with oodles of bagged topsoil.
Tractor supply had a sale on small plastic swimming pools, so I got one ($10) and I thought it would be deep enough for a "field trip" for the youngsters (2.3 weeks) I thought about cutting the bottom out...then place the bebes inside. OR putting a dirt/bathing mixture in it, and let them cut loose...especially since I have a punk bebe.
For coccidiosis exposure, best to use soil from somewhere near your coop/run, as that's going to be what they're naturally exposed to. Even if it's clay, I'd just dig up a clump with maybe some grass and roots if possible, and that should be good enough for them to play with.
For dust bathing later on, if your soil really isn't suitable for that, you can use topsoil, peat moss, sand, wood ash - mix and match any or all if you desire.