Mila...very interesting topic. There's a big difference between a PPD, and what you want IT to do and a sport or PSD, and what they do. This difference is hugely accentuated if you train a PPD using only defense.
PSD and sport dogs sometimes need to cover a bunch of ground, or they need to use their nose to "catch" their PREY.
Defense ONLY works in close. That's it...only in close.
If it's a long bite...defense may (and should, along with fight drive) pick up markedly when the dog is on the man, but it's not the main driver during pursuits.
For personal and close protection, though, decent bites are possible to obtain with the majority of decent working dogs in defense...and that may be all you want.
To teach a dog to find a man in a building and then bite him you'll be using a bunch of prey drive...even if you don't think it's apparent. It's prey drive.
Some...very exceptional...dogs will take the fight to a man that's far away. Not many of these are available. Most will use prey, and a history of winning confrontations, to go long distances.
Then there's the issue of teaching a dog HOW to fight. You've seen it in the wild a million times. All predators play fight. This is very important. The only way to play fight with your dog - to build him up - is to use prey drive. This drive helps overcome stress due to obstacles, surfaces, multiple attackers, dark, confined spaces, elevation, off positions, etc., etc. THEN you get into the real deal, with a trained fighter.
You can also do it with defense, but it will take longer, you'll stress the heck out of your dog...that is, assuming your dog can take it. There is a tremendous attrition rate with this method. TRue, that if you find a dog that will do all that, at a distance, using fight drive, not defense, you'll have a very exceptional dog.
I have never seen one that doesn't switch into prey during the pursuit.
Finally, and this is important, dogs don't transfer biting (with good decoys, that is) a fast moving tug or sleeve...which is what they see as "prey", to biting running children. Dogs that will chase and bite running children...or everything else, for that matter, have that instinct IN them...from birth and genetics. You can and should extinguish that BEHAVIOR, but if the drive's in the dog, in the short run you will not eliminate it. You can only SUPPRESS it. Obviously, as I said, you can extinguish the behavior in the long run. This association between tugs, sleeves and children does not exist. If a dog chases a tug, it will also chase a child. If you teach it NOT to chase a child, and you teach it TO CHASE a tug, it will only chase tugs, and so on.
So: 1)I think it's very incorrect for dogs to be trained to bite using only defense, because they are unduly stressed, 2) Practically all dogs that will hunt a man for a bite, will do it in prey anyway, 3)Teaching dogs fighting technique using prey is a million times more effective, and 4)Using prey for training doesn't automatically translate to "Bite children that are running". That last part was an ingredient in the soup to begin with.