You have mites. It's not uncommon, and nearly every flock has some degree of infestation.
Pyrethrin powder over the hen's back, and it will filter down to the chicks and take care of their problem too. It's very safe, and works faster and is longer lasting than the DE. Sevin powder works well also. The DE is great for a preventative measure, but you need to kill these little buggers fast! Otherwise, the mites will sap the strength of your babies.
You will also have to use it on the other chickens in your flock. Also clean everything, and spray a safe insecticide on your nests, roosts, cracks in the walls, floor, ceiling, etc. Good luck!