For the price variation in the same breed of dog, there is pedigree and non-pedigree dogs.
Pedigree dogs are dogs with known bloodline to at least 5th generation. Apart from being registered, you can easily identify the genetic structure of the pedigree dog looking at the bloodline. This will give you information about the temperament, body structure, head formation etc of the dog even at puppy stage. That is, you have an idea of how your dog will be.
Non-pedigree on the other hand is the opposite of the above. There is no record to show. Majority of the puppies will look good but loose shape and formation while growing up. This can be as a result of the breeding pattern of the parent dog.

There are notable difference between the two pictures of dogs that I shared yet they are called Caucasian

This is H-Cube Kennel. This link was sent to me by a brother.
For security dogs, you can get Caucasian, GSD or Rottweiler. Caucasian dog is a large dog and barks very well. GSD and Rottweiler can serve the purpose also but they are not as big as Caucasian. The temperament of a dog depends on how one raise the dog. Even if you are buying a puppy or semi-adult ( between 2month - 6month), the temperament of the parent of the dog you are buying matters. If the parent are extremely wild dogs , then there is likelihood the the puppies will be.
And introducing puppy or semi-adult to children is very easy. If the temperament of the dog is cool, allow the dog to play with the children under your watch.