This is common among almost all of my clients. What you need to do is several things....it is obvious that when a dog nips, natural reaction is to pull away. If it is not serious, the best thing to do is to either push your hand/arm into the mouth, causing them to release OR make a loud "OUCH" or yip noise that gets them to let go. By pulling your arm back, you are taking an almost submissive position and the dog has won, probably going to try it again. What probably got one dog to stop is the high-pitch noise.

Make sure your children do not play with your dogs face - like batting it around. You also want to avoid playing near their mouth with your hands - always use toys. This will help resolve nipping. If you play with them with your hands, all your puppy knows is that your hand IS a toy sometimes.

Make sure you have plenty of toys and always redirect his nippy behavior with them. Rubber type toys are the best as they are basically shred-resistant.

Any questions or further advice, let me know.