There could possibly be a
riboflavin deficiency in the diet of the parents, though curling usually starts younger--at a few days of age. If this is the case, my understanding is that extra riboflavin in the chicks' diet still mighthelp them recover.
I looked up the supplement you're using & see that it does list Riboflavin as a significant ingredient, so it may be helpful. If problems continue, you might try another supplement. Or adding some high-Riboflavin foods, such as paprika, dried coriander, spearmint, parsley, ground almonds (you can grind these with a hammer), dry roasted soy (NOT fresh soy), cheese, ground sesame seeds, etc.
Riboflavin is also degraded by exposure to light, so be sure your feed isn't stored in light, & also isn't more than about 3 months old (Most bags show the milling date on the bag).
Be careful about keeping or selling ones with curled toes. From what I'm reading, they generally die within a few weeks from their feet being so painful that they stop being able to walk to food & water.
There is a little info on treating Riboflavin deficiencies & chick shoes & other treatments that may help on the
Poultry Page in the website linked in my sig below.
I don't know if Riboflavin deficiency is for sure the cause of your chicks' toe problems, but I hope you are able to determine what is & find a solution.