Although you didn’t give me your daughter’s age, I am going to guess she is 5 to 7. Children rarely pick at or bite their nails, before that time.
I don’t ever want to minimize a parent’s concern, but I wouldn’t worry about the actual nail-picking or biting. Your main concern should be why she does it. There is very little harm that can come to a child who has this habit, although sometimes they will draw a little blood around the cuticle. They usually know when to stop, however.
This is an easier habit to deal with in girls than in boys, as little girls love to wear nail polish. So play “Let’s Make a Deal.” The deal would go something like, “If you go for a day (later two days, then a week, etc.) without picking at your nails, I’ll let you choose the color you want and polish your nails for you.” If she comes home from school giving evidence that she violated her part of the bargain, then the polish remover gets used that evening. Some kind of reinforcement like this usually will take care of this habit.
A final word: I’d be more concerned about why she picks her nails than that she does. It may just be a habit that can easily be modified with some version of the above procedure. But it might indicate that she is anxious about something that she won’t talk to you about. If your “deal” works, make sure you stay alert for other signs of trouble.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.