This is where you get a little more serious about working with the puppies and cultivating the right personality. Now that they can see and hear, they will be much more interactive and impressionable.
Teaching Puppies Not to Nip
One of the reasons why puppies need to grow up among a litter is to teach them manners. As they interact with one another, they learn that their teeth are sharp and can cause harm. This teaches them to be careful when they play so that they don't bite down hard on each other and cause injuries.
Important: When a puppy plays, they are using their instincts to prepare for real-life scenarios of defending themselves or their pack (you). It is the same reason why you see lion cubs playing together. However, they also learn to discern that practice time (play) is not the same as real fighting.
When a sibling yelps because a puppy bites too hard (even at a very young age) they will stop and play more gently. They know that this is play and not a real fight, so they aren't actually trying to hurt each other.
This is very applicable to your dog when it is being trained for work as a protection service canine. You can further teach them to be careful with their teeth using a method that I've discovered in breeding my own personal protection service Dobermans.
Take a huge slab of meat (like ribs or a ham) and just throw it in the kennel/whelping bin for the puppies to eat. (I always separate the mother for this so that she doesn't eat their food.)
What will happen is that as the puppies eat, they will accidently bite another puppy's paw or get an ear and you'll hear that tell-tale yelp. Don't be alarmed, this is actually a very good thing. What I've discovered is that the puppies will self-correct, stop and be more careful about where they put their teeth. As a result, they will be more careful of you (and people in general) when you hold out food for them or play with them.
- They basically taught themselves not to nip!
This is also good for them because it slows them down since they have to rip the meat off of the bone themselves, and it constructively directs all of that energy and teething that they are experiencing towards something useful.
Side Tip
When your puppy is old enough to begin training, hold the treat out in a loosely closed fist. This forces the puppy to lick the treat out between your fingers. As a result, it'll naturally prevent them from using their teeth to grab the treat and accidently nipping you.