The following is just a train-of-thought post, with some info. for you:
I have to treat at least every 4 months or more with permethrin otherwise we get infestations from the wild birds.
If they have never been outside then they cannot get mites. Diatomaceous earth doesn't treat an infestation.
I would set up a dust bath for them with some sand or dirt out of the rain and that will help them keep bugs away. You can put some DE in the bath too- it might help some.
Be sure to spray your coops as well. When you treat for mites, do two treatments, seven days apart due to the life cycle of the mites (if you have poultry lice then do a third treatment as the life cycle is longer, or just two treatments two weeks apart). I use poultry dust for this, and make sure you get under the wings, around the vent, and don't dust the face.
Baby chicks don't require dusting unless they already have mites. Mites will bite humans but cannot reproduce unless they have bird blood. The Northern Fowl Mite lives for 3 weeks with no blood meal and the red mite lives for up to 9 months with no bird blood meal. So if you collect them in the home and then they are gone off your chickens, simple vacuuming will get rid of them (and changing sheets etc.) eventually.
But on the chickens they will reproduce and eventually even kill some of the chickens from blood loss. So it is a part of keeping chickens, the keeping away of mites (or treating when they do get infested). Look for mites around the vent and under the arms. I never have seen them on the chickens though- I find them on me or biting me- that is how I know when they are infested.
You can also put the poultry dust in the dust bath for them but they will eat it and I don't like that so I dust them with a tied-off sock (powder inside it), turning them on their backs and holding onto feet on ground, then flip them over.