Having dealt with two very severe resource guarders in my life, I can tell you that you want to get this under control now, before your puppy is much larger.
Step #1 in dealing with resource guarding is understanding that you *cannot* fix resource guarding by taking things away from a dog. You will *not* teach the dog to not guard by removing objects from the dog. In fact, that reinforces the idea that every time you approach, you take something away. I'm recommend a multi-step process to working with Cinder on guarding.
1. The most important step is to make sure that your son NEVER forces Cinder to bite him because he ignores her warnings and continues to approach her. Cinder did not deserve a time out for that situation. Your son approached and she growled, which was her saying "I'm uncomfortable, I don't want to escalate, please back off". Her verbal communication was ignored. Ignoring a dog's warning is like ignoring someone who says "please don't come near you, you're making me uncomfortable" and then approaching them anyways, and then being surprised when you get punched in the face.
2. As Karen suggested, build a positive association between your son and Cinder by setting up activities where he can walk by and reward her while she has a high value bone. No interaction needs to take place other than she needs to be laying on the ground chewing, and your son walks by, calmly drops a treat near her, and walks away. He can do the same thing while she's eating her food. The idea is to let her know that your son being nearby means good things happen, but not having him make her uncomfortable by invading her space or threatening her resource. Walk by, drop food, leave.
3. The other thing you should be doing is playing trade up games. Give Cinder a bone. Get an even higher value treat (cheese, small bits of meat), dangle it in front of Cinder's face while asking for a "drop it" and then reward when Cinder drops the bone. This will give Cinder the impression that every time you're taking something from her, she's getting something better in return.
4. The final thing is this - NEVER punish your dog for growling. Growling is a really important communication tool. Dogs don't have words, so any warnings we can hear should be taken as vital pieces of information.
To illustrate, this video shows a dog who has had the growl trained out of it. The dog is clearly uncomfortable, has no way of communicating the discomfort, and then snaps with no warning whatsoever: [video=youtube;rFVFq1kAhc8]https://www.youtube.com/watch?v=rFVFq1kAhc8[/video]
You don't want a Mastiff who feels like it is incapable of using a natural means of communication with you. Respect growling, figure out the source of the growling, and fix the source. Fixing the growl doesn't remove the discomfort, it only removes the unpleasant noise and leaves an uncomfortable dog with no means of defense besides biting.
You can overcome the ant hill that Cinder's resource guarding is now by working diligently to make sure she understands that you are not a threat to her things. However, if you ignore it, that tiny ant hill could turn into a mountain. I have large and permanent scars from my very severely resource guarding dog. Especially with kids in the picture, it's a problem that needs to be tackled asap.
