Yesterday I helped my cousin with her new chicks she got on a discount from the store, that are the same way as yours. We checked them over and found mites so we dusted them up careful around the head, used small brushes to apply the powder. We cleaned the butts and had to trim off some fluff some were bad with red irritated butts, neosporin put lightly around vents. Cleaned the balls of cemented poo from the feet, 2 are nearly crippled, we had to make shoes from foam to straighten the toes back out. The poo was making them walk like they were wearing stilletto heels. They weren't very interested in the feed or water.
I ran home and got poultry nutridrench to dose up the chicks, and the electrolytes and probiotics. We gave each chick a couple drops each of the nutridrench, and drops of the treated water into their beaks. Within a half hour most of them snapped out of it and began devouring feed like crazy. The two that had the worst feet and butts were more energetic but needed help walking on the foam feet to get used to them. When I left the chicks were wobbly but standing and eating.
She is getting the poultry nutridrench, and some other chick vitamins today. Besides giving the store manager a piece of her mind on the condition of the chicks. If she hadn't taken them I am pretty sure they would have died in the next day or two. That dirty of litter for a chick is a death sentence. They may not be able to watch the chicks very closely at the store but it is obvious the litter needs to be cleaned out once in awhile. Heck a lady was asking to buy the used litter for her garden one day when I was there!
Hope your chicks recover and thrive. Good luck!