I am surprised the 2 dogs you already have didn't make enough noise to run the intruder off. Perhaps the Lab can't hear anymore. I am sure my 14 year old Lab would alert us to anything that woke her up. Your research should have suggested to you that a dog trained to do much more is a specialized area. That is outside my experience and training. I really would suggest professional help if you go with that.
With the 8 year old and friends, I would stay away from the breeds you mention and anything remotely related to Pit Bulls. Look at other large breeds to produce a deep bark suggesting a dog not to be messed with. The kid that gets bitten because it mistreated a Golden likely will faulted for his misdeeds. Let a kid provoke an American Bull Dog to growling or showing it teeth and you will have a great hue and cry about a vicious Pit Bull. No, that isn't fair, but the way our society works.
Your suggestion of a Mastiff might be a good idea along with a standard or giant Schnauzer. All 3 were bred as watch dogs, but don't have the bad image many other breeds do. Unfortunately, you can't count on any dog being true to its heritage. Many of my Labs want nothing to do with swimming. Ordinarily they love any chance to play with other dogs, but wouldn't join them in the lake.
One of the best sources for dogs with a predictable personality is the rescue
dogs. These are dogs that lost their home, but were taken into a foster home
to be retrained as necessary and placed in the right home for them. You may
find a rescue near you starting at
American Kennel Club - Breed Rescue The rescues charge a fee to help cover their expenses, but is much less than the price of a puppy plus all its medical expenses the first year.