A.R.
My son (8 yr. old) also picks at his nails and his fingers. (He has ADHD and some OCD tendencys) We use silly puddy at school (he doesn't play with it, just rolls it around in his hand), which helps a lot, he had picked a hang nail all the way around to the pad of his finger and was drawing blood, which has been gone for months now. A worry rock (smooth rock they can rub or roll around in their hand) would work also. It gives him something to "distract" from the picking.