CHEWYS
Some children (as well as adults) have a strong need to chew or bite on their nails, nuckles, pencils, shirts (sleeves/neck). It can be very hard to replace that need with something more appropriate but there are more and more items on the market that can be explored. Just make sure they are safe in terms of what they are made of and that they hold up for extemisive chewing. Here are some examples:
Chewable Jewels a necklace and a bracelet. They are lead, latex, pthalate free and dishwasher safe.
This chewy can be placed on the pencil but it can also accommodate a mini-carabineers and be clipped to a belt loop for quick access. It is available in regular or extra tough version with or without bumps.