Have you spoken to your local humane society? If they aren't able to help, they might have advice.
In the meantime, if the cat is chewing its legs, you need to get a cone on it ASAP. If it causes enough injury to merit an amputation, they will probably recommend euthanasia instead.
They probably need wound care and antibiotics if they have open wounds, but that will be a challenge if they are wild.
Unfortunately if the mother is not caring for it, that usually means the mother has decided the cost of caring for it is too great at this point.