There are different types of mites. Are they on your birds 24/7 or just at night? If just at night, it is most important to treat the environment than the birds and find where the the mites are hiding during the day on the roost and in the coop.
Scaly Leg mites are only on the legs & shanks, make the scales pop out & your chicken will limp. With an eye dropper or syringe, coat the legs and shanks with Campho-phnique being sure to even get between the toes.
I've recently dealt with a mite infestation which appeared seemingly overnight and they exploded very quickly. They were the knid that are on the birds 24/7: probably Northern Fowl Mite. Check roosters first as they are always the most infested. Treat the least infected birds first so as not to transfer mites to them. There are two kinds of sevin dusts: Sevin 5% and Sevin 10% (active ingredient is Carbyl (m.s.?). You are supposed to use the sevin 5, but I will go ahead and tell you to use the sevin 10 (as mites have become more resistant).
Wear a dust mask yourself if you can bear it. Dust the entire bird turning each UPSIDE DOWN making sure the sevin gets all the way to the skin. Try not to get it in their eyes. have someone help you. Massage the sevin deep into the feathers so as to work it all the way down. Cover them with sevin except their eyes and nose. You might have to pull off scabs or the patches if infestations are bad. Dust around the vents, under the wing, at base of tail & back and on the neck real, real good. The longer you wait, the worse it will get and fast. REDUST IN 7-10 days! If this does not get rid of them, post again & will take another direction.
Change the bedding in your coop and the litter in your nesting boxes. Put sevin in the floor of your coop and in each nesting box & mix in litter. Just wash your eggs with water, of course, that is warmer than the egg (I do with hot tap water). You can also paint mineral oil on the roosts. to suffocate any mites on the roosts. GOOD LUCK!