Oky i need more help dont wont to give up on her yet!!?

i posted a whiel ago about my 11 week old mastiff , ahs has only done this 2 times my 2 year old fell on her and pushed a cheir into her, my son should have,not done that he got in to truble , but she got mad and went after my son !! i dont wont to just get rid of her for that is she hurts him or dose it again i will do what i have to do my son is by far more inportant , than a dog!! but we have her in puppy school for traing, and all of that stuff . can we do something to help her not to do this??????????? i love her and my children ! what do i do???