Its good to have a guard dog around. I know what you are saying, it feels good to know your dog can protect you on walks and stuff.
But you should be in love with the breed too. I am getting a Cane Corso, the only reason not a regular Mastiff is because they are a bit smaller and they are more protective. They were bred for gaurding, not fighting. Not that fight dogs can't be good companions with proper obedience and stuff.
My boyfriend has had chows which are very dominant dogs as well, its important you can handle a large dominant dog, or it will try to dominate you.
The best way is to eat before it, and when you feed it have it sit and wait for you to say okay it can eat. Put your hands in its kibble and touch the food dish when its eating. If it ever growls flip it (don't hurt it) onto its back and rub its belly. you can also shake the nape of its neck on the back gently, not too hard, this is what a momma dog does. Flipping it and rubbing its belly firmly is the best way to show you are the "top dog". Don't rub too hard tho you don't want to hurt your baby.
Always enter the house first, and lead on walks. Don't let your dog go first.
Those are basic ways to have your dog in control. Also make him do something for his treats, like sit or stay or lay down. Train your kids to do the exact same and you should have no problems. Take him to puppy daycare as soon as you get him and take him to dog parks regularly.
A good protective dog will NOT be agressive with people you are okay with. They should be able to sense weird situations and will only become protective when its actually needed. You should not ever see agression unless the situation is happening. Like someones trying to mug you or rob you..
Your dog should be ALOOF with strangers but warm up when you let them know they are okay. They should never growl at children or be agressive with strangers. Aloof is alot different.
I was scared of my boyfriends chow, because i didn't know him as well when he had him. He never growled or tried to do anything weird to me but i could sense he was very protective, especially of his children.
Anyways, good luck on your search remember you have to socialize the dog VERY good and keep control of it!!
Also, dont let it sleep on your bed, it will think you are another dog and that can cause lots of problems.