I've struggled with mites on my banty cochin. None of my other birds have gotten the darn things.
Go to your local big box home store, Lowe's, Home Depot, or a small hardware store, and get some Seven Dust 5% in the garden section. It comes in a cylindrical container that's yellow & red.
Along with the Seven, get a dust mask.
Grab yourself a chicken, hold her down, and dust liberally. Lift her wings, push feathers back, and make sure you get the vent. Do all the chickens.
When you're done, thoroughly clean your coop. dust the coop, paying close attention to the roost bar(s) and crevices, where mites like to live.
You'll notice that all the mites will be gone in a day, but you're not done.
You'll have to re-dust the chickens in 5 days if there are more mites. This is because Seven doesn't kill eggs.
I had to trim the leg feathers on my banty. She's always damp b/c her legs are so short and it's muddy here. She get anemic really quickly when she has mites b/c she's only 20 ounces.
Good luck & keep us posted!