The video below is toenail grinding on an 8-week old puppy.  You can hear the puppy crying but he is obviously not being harmed in any way.  I have him stuffed in the chair beside me and am controlling him by pulling him up close to my body and hugging him with my arm.  It is best for young puppies to learn to deal with grinding when they are young.  You should grind all toenails on all four feet on a weekly basis to teach the puppy to deal with this sort of grooming.