|A female Scarlet-chested parakeet.|
If you enter "handfeeding" into the search window on this site, you will get other posts about feeding baby birds.
Since your babies haven't been fed enough recently, I'd offer them warm food every two hours during the day for the next few days. I'd even be tempted to get up in the middle of the night for the first day or two, just to get them healthy again. Try to feed them until they look like their crops are swollen and large. If they shake their heads, and refuse food, they're full. If they want to keep eating, let them.
If your birds are outside, even in an enclosed building, it's possible that your hen threw the dead baby chick out of the nest and a rodent found and carried the body off overnight. They can get into spaces you'd never believe possible, and are attracted by bird seed.