Ultimately what sparked my interest in specifically a guard dog is that my children like to play in the woods immediately behind my house. While we can see and hear them from the house and back deck there is a risk. I was in the woods with my family the one day and a dog came on the other side of the creek barking at us. Now granted the tail was wagging and it may have been friendly but you never know. It was frantically trying to find a path through the brush to get to us but was unsuccessful. It made me think when if the kids were at there one play spot where the dog could have got to them. Like someone said it only takes two seconds. It would be nice to have a family pet that could provide some form of determent.
Also, I am unarmed. If an intruder broke into my house I would possibly have no way of knowing and be defenseless in protecting my family. I have been considering options and this would be just one step in additional security for my household.
I just so happen to live in one of the safest cities in the country (Cary), but I feel it is wise nonetheless to be prepared to protect the family.
Since you seem to be planning to get a dog that would be basically alone with the children if when they are in the woods, I would say you also have to get a breed that's known to be good with kids...
German Shepherd
Great Pyrenees
Rhodesian Ridgeback (able to keep a lion at bay) (good for beginner dog owners)
Mastiff
However, since "guard" dogs are trained to hurt people, and since they react to quick movements, you do run the risk of your children getting hurt by the dog as children are known for the quick movements, and for even stepping on a dog's tail and whatnot.
If you get the dog, it might be a good idea to have the kids involved with the training. That way the dog will hopefully recognize that the kids are above them in the pack. Problems can occur if your dog thinks it has to climb up the pack ladder by making the kids submissive to it.