Hi, try putting nail biting varnish on, or piri piri. My kids sometimes do the same, and I am so worried about all the germs that they may be transmitting to their system, that I put the varnish on and it stops them whilst its on.
It is likely this behavior began as a means of stress/tension reduction and developed into a habit pattern. You can stop it via a combination of cognitive-behavioral therapy and perhaps medication. This Forum addresses child issues, but you should post your question in the Mental Health Forum for additional guidance.