Mites can kill a chick quickly. i've never had chicks come from hatcheries with mites. Are you sure they have them? Have you seen them? Pick up the chicks and check carefully. If they have them they will probably end up on your hand.
i had a fairly wild hen who hatched out two chicks. Didn't realize she had mites and they quickly infested the chicks and killed one within a day. The other chick was badly infested also. i had to take it away from mom. Didn't want to use poultry dust, so i used the Poultry Protector spray. i sprayed it on my hand then held the chick for a while until some of the mites came off, then put it under a heat lamp to dry off. Repeated this many times over the course of hours. Eventually got all the mites off. Then seriously treated mom and dad with poultry dust, ivermectin drops on back of neck, and thoroughly disinfected their coop and pen. Although i did end up raising the chick in the house.
Check those chicks and make sure they don't have mites. It isn't something they can easily survive.