I know ALL ABOUT your chicks' problems if they are from mold!!
I nursed 2 chickens for months with internal fungus problem that came from my feeding them moldy corn.
This is so hard to watch & try to help. I did not know for sure the problem, nor the treatment with my first chicken. I didn't know correct treatment until very late with the second. They both slowly died of starvation.
Unfortunately, the condition is EXTREMELY hard to cure and has a very poor prognosis. It may be even harder on your guys because they are very young.
I will create a web page with information on treating this condition, and send you a link. I will try to post as much as I can get up within the next day.
Meanwhile, some important basic tips:
Carbohydrates and sweets *feed the fungus*, which then does more damage to the chickens' insides plus releases more toxins. Do NOT give any more molasses, sugar, etc.
Proteins are great. Grasshoppers, earwigs, mealworms (though very expensive), freshly hulled nuts other than peanuts (pre-hulled nuts are more likely to have mold on them, as are peanuts), meats of any kind (fish, pork, beef).
It is okay to feed some regular feed, though it has quite a bit of carbs. Also do give other foods to help balance it out, though.
A little unsweetened yogurt is helpful to nourish good bacteria that are getting killed off competing with the fungus. Don't overdo or they will get diarrhea.
Add vitamins and mineral powder to their water. (**Do NOT add this to food. Chickens don't like the taste. They will still drink water fine with it added, but will avoid food if you add it to that.***) Add a little more than the label's directions. A pouch Vi-Tal or similar supplement at a feed store costs only $4. There are electrolytes in those, which means there is some sugar, but hopefully not as much as in Poly-Visol children's vitamins, etc.
Many vegetables are good for chicken's struggling with this--spinach, pumpkin, lettuce (other than iceburg kind). I don't remember for sure about tomatoes... I'll post more info on webpage when I look it up.
I would keep the mom and chick's in a restricted area so they don't have to travel a lot, and so they have ready access to all the food and water they will take in. You might even want to add a low-volt light bulb in a heat lamp in the area, if the chicks have a hard time staying under the hen & seem to get cold. They need as much of their energy saved as possible! The fungus is eating their food, poisoning them, and potentially damaging the walls of their digestive organs.
There is a hard-to-find chemical that helps treat this. I have it, though it might possibly be stored in another city where I can't get it until next week. I can mail to you as soon as I can if you PM or email me your address.
I'll give more info as I have time.
Best wishes!!!
p.s. Since your older chickens had access to the moldy feed, they VERY LIKELY have the condition, too, just are not showing signs as much yet because their systems' are stronger. Give them appropriate diet, too. They probably are okay not being confined as much, though, unless it's cold.