If a child bites his fingernails and eats them, is there any harm?

Well, yes, there is.: Nail biting causes chronic infections of the nails and the skin around them, and eventually can lead to permanent scarring of the nails. Conversely, fingertips are loaded with bacteria, which put you at varying degrees of risk when introduced to your body through your mouth. It's not only unsightly but unhealthy. Stopping is not easy; entire books are devoted to techniques for breaking the habit.

Yes and no: The nail itself is not toxic but the practice might introduce germs into the body that could cause illness.If done to an extreme it would injure the fingers and provide an open wound for germ to enter and cause nailbed/finger infections. In most kids/adults who randomly bite their nails, their bodies defenses are adequate to prevent any damage.