Personally I would not use hemp. I'm not sure of the official status on if it's safe or not, but it's not a plant I would give my chin. And I'm not sure if the stuff in the stores has any sort of oil on it?
If you can get oil free sisal rope, that is safe. As well as coconut rope.
I wouldn't use hemp because the individual fibers aren't very stiff so they can be swallowed and possibly cause a blockage. Sisal would be a better option and would last probably as long as the hemp would with chins chewing at it.