Find the worst tasting hair gel you can? It works for fingernails.

Otherwise, I would tentatively suggest visiting the GP and asking for a referral to a child psychologist. I hate to see children labeled at such a young age, and I don't think that is what will happen. It sounds to me like a very mild form of OCD, which believe it or not is not uncommon in children at that age. A psychologist might be able to give some advice about strategies to distract her and break the habit.