Nail biting is a common scenario amongst children. It has been associated to various psychological states. It seems it may be a trait linked to a nervous state of mind, anxiety and so on. But we rarely think about the repercussions of biting nails with a physiological angle.
According to the Nemours Foundation, chewing nails down to the cuticle can cause bleeding, which exposes the skin to germs and increases the risk of infection. When your child puts germ-laden fingernails in her mouth, this also increases her risk of illness.
You can explain the ills nail chewing does to your child by speaking to them. If that does not work you can ask your doctor or pharmacist about special nail polishes that have a bitter taste and may discourage the practice.