Laborer Biting aggressively?

We have an 11 month old black lab. We got her when she was four months old. She has been a great dog around the kids. She has never tried to bite anyone or show any bad behavior. She is a inside dog most of the time. My son has always messed with her food since we had her. We have tried to tell my son no no that's the dog's food. But he doesn't always listen. Anyways.. That has never seemed to bother our dog at all. But today my son went over to her bowl and got a handful of food to give her and all of a sudden she aggressively attacked him. She bit his face. His forhead has a goose egg on it and is purple with two teeth marks that bled all evening. She bit through his bottom lip causing alot of blood and swelling. When she attacked him she literally knocked him off his feet. We really love our dog.. But never want this to happen again. And whats so bad about this is we actually got the dog for our ten year old daughter. But the dog seemed to take more to my son. They are like best buddies. They play together,Sleep together My son even shares his food with our dog and his toys. Please can anyone give me some advice or insight as to what happened and if you think it can happen again. Thanks