i've never heard of using the dip on chickens. It is poison and too much will kill anything. i think even the spray is pretty pungent. When i have a bad case of mites, i will give the chicken a lukewarm bath with just a couple drops of flea shampoo in it, then do several rinses. But my routine is a few drops of ivermectin on the back of the neck (number of drops based on size of chicken), then use poultry dust on their body - carefully avoiding eyes and face.
Were it me, i would bring the chickens in one by one, worst cases first, and soak them in lukewarm to warm water to rinse the poison off. Then blow dry them before putting them back out (low setting, careful to keep your hand there to make sure it's not too hot. Then maybe some vitamins in the water and nutritious treats to bolster them - like mashed hard boiled eggs.
Hope they make it!