This will probably require a little training with the puppy from you, and practice for your twins. My approach has always been to teach the puppy that biting won't get him a treat by holding the treat in a closed fist, if he bites he doesn't get it, but if he licks or just pushes his muzzle at your fist you open it and let him get the treat.
Teach the kids also to hold treats for the puppy in a fist, and only open their hand, palm up of course, if the puppy isn't biting. It takes a little practice, but it shouldn't take long to teach the puppy to take treats without biting.
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain