As said, she should be fine on chick crumb which should build her back up. Feeding young chicks layers pellets may be fatal because of the high Calcium levels. They are too young for rearers, which they shouldn't switch to until 6-8 weeks.
Darkening of her face may be an early sign of a heart condition Bumpkins. Clearly something isn't right. She will be working hard to raise her body temperature to keep the chicks warm and it takes a lot out of them. You may need to separate them early and give the chicks some heat and best to have that setup ready.