I agree with other posters here, that a guard dog is not a family dog, especially if has been raised and trained by someone away from your family and doesn't know you and your kids from a very young age.

We've got a lab. Apparently the perfect family pet. He's brilliant with DD, he's never given cause for concern around anyone we've let into our house. He's been beautifully trained by us, but even he has an instinct that I cannot control. At our old house the electricity metre was in the laundry. The metre reader just let himself in one day and our dog let him know that wasn't acceptable. Since then one of us had to be home to have the metre read as he was listed as dangerous by this man. If someone ever comes into our backyard uninvited the dog runs at them barking, he does this stomping run that sounds like thunder. He never gets close but at the same time the person always goes straight back out again.

I guess my point there is that you don't need a specifically trained guard dog to have a dog that will protect your home. I would be very concerned about adopting an older, unfamiliar, already trained guard specific dog with my kids around (and honestly, I probably wouldn't do it if I didn't have kids either). I think the point someone made earlier is also good - they listen most to the one that trained them. We had a German Shepherd growing up, and while he was good with my mum, brother and I, no one could control him like my dad. He was the pack leader.