If you have a moveable pen, put them on concrete(driveway, maybe?) and then put a little bit of scratch mixed in dirt or straw and then spread it across the concrete. The idea is to get them to scratch around in the dirt looking for tidbits of food, but because the layer of dirt is so thin they end up scratching around on concrete.
The concrete with file their nails down pretty well. It is why dogs kept on concrete have the best nails.