My family dog is an 85lb female boxer. She is EXTREMELY protective of my daughter and all her little friends. It is a function of her breeding, as boxers were bred as herding, and protection animals for livestock. They are tough and hardy, built to handle wolves and as such, do well on softer flesh...like humans who threaten their kids. Female boxers are especially protective of children. My 5 year old daughter has a permanent stubby tailed fawn colored shadow.
When there is a noise outside, the dog alerts, and if someone steps onto our porch, she goes into what I call LOUD TERMINATOR LANDSHARK mode. When I get up, she quiets down to a low, steady growl as I answer the door. She is very sweet, but with the flat, black face, she looks menacing and hazardous. Boxers have none of the so-called "dangerous dog" reputation that you will get with a rottie, sheperd, chow, dobe, etc...
As a note, female boxers have a more dominant personality than male boxers. My female had to be taught from day one, that I was the alpha in the house, followed by my wife and my daughter. This isn't hard to do, if you are willing to take the time and form the habits needed to make it stick.
NEVER punish your dog for barking at a door knock, or doorbell. Get up, and go to the door, and then praise the dog for doing a good job. Then YOU, as the alpha dispatch the "threat". The lower pack member's job is to alert you, that's what they get paid for.
NEVER let your dog lead YOU through a doorway. You claim it as YOUR doorway, everything in your house is YOURS. The alpha ALWAYS leads and chooses the direction of travel.
NEVER let your dog win a tug-of-war with their chew toy. When the game is over YOU keep the toy and put it away. The alpha chooses playtime.
When you feed, make sure you put the food down AFTER the family has eaten. The alpha chooses when the rest of the pack eats.
NEVER walk around your dog. Make the dog move. The alpha doesn't move aside/around ANY lower ranking pack member.
NEVER pet or praise the dog when it is in an excited state. Make it calm down, and you choose when it will get affection or praise.
NEVER go TO the dog to give affection, call it to you and then give it affection. When you call the dog for praise, affection, or feeding, use it's name. NEVER use it's name when you are calling it for corrective action.
You don't need to train a dog for protection, their natural instincts will generally take over when it is time to do it's part in protecting the pack. I have trained my boxer to go into her crazy barking hackle up terminator mode on verbal command from me. It is one heck of a good deterrent, or distraction of I have to draw my weapon.