They're not supposed to be fat, just feed them their chick starter with egg yolk mixed in. They'll catch up. I know it's rough but you kind if have to 'harden your heart' to chickens, they can have life threatening issues at any age and are on everybody's menu.
That's a pity. It sounds more like intestinal shed but it doesn't hurt to treat Coccidosis.
Baby/chick vitamins will help and you can mix raw yolk into the feed.
What's the symptoms with your birds? What's their set up?
Egg yolks are more nutritious than the whites for babies but their chick feed should be be fine without and add ins.
Raw yolk is messy.