Soak the feet in soapy water, then gently scrub with a soft brush to remove dirt and debris. Apply Vaseline, A+D ointment or something similar to the legs and feet. Apply your ointment daily for a couple of weeks. This will smother the mites and help heal the feet.
Scaly leg mites generally live under the scales of the legs and feet, but can be transmitted through contact and shedding of scales.Since you have several that have the same problem, clean out your coop and treat with a permethrin based spray or dust - this will hopefully eliminated any that may be in the coop. Repeat in 7-10days.
Try to keep bedding as dry as you can. Offer some poultry vitamins a couple of times a week to give them a boost while they are healing.