Supplying them with a really good dust bathing area usually helps prevent / treat them really well, one that has a bit of ashes in it is really good.
I would clean the coop out of the bedding, the nest boxes cleaned out as well as the roosts cleaned (the mites will hide in the wood of the roost and climb back onto chickens).
Then when reapplying all the bedding/nesting material sprinkle some diatomaceous earth (it's a white powder) you can usually get from most feed stores. This will suffocate the mites hanging around by clogging their pores with tiny particles. You'll maybe need to do this a few times to be sure you get the life-cycles of the mites, you might be able to get away with just a re-sprinkle of diatomaceous earth vs. the whole entire coop cleaned out.
As for the actual treatment on the girls themselves...i'm not entirely sure when it comes to body mites.
Under the learning centre there are some really good pointers, i'm sure there will be one specific to body mites under there.
Hope this helps a little bit, and that they get better quick!
