Why would my puppy bite someone?

my dog is 8 months old, he is mastiff and pitbull but more mastiff, i just cant believe it! it was my neighbors daughter which is also my boyfriends cousin. i was not there when it happened but the story i heard is that my cousin (4 years old) was sitting on his back and hitting the little girl so im guessing he was protecting my little cousin but he should know not to bite a little girl. what should i do? && how can i talk my mom into keeping him because she does not want him back in the house cause we do not know if we can trust him with kids now but he never acted like this && i do not want to get rid of him i had him since he was 3 weeks old! he had bite her in the arm she only needed 4 stitches and he has been around alot of people and children since the day ive gotten him.&& also i had him since he was 3 weeks old because the mothers milk dried up so i had to bottle feed him & everything i love this dog with my whole heart hes part of the family && i do not want to give him away now! a friend of mine said he will take him but should i be worried how he will be with kids now?