It's simply doesn't make sense to have an animal remain obedient to, and chaperone the very child that he also wishes to dominate. One myth about dogs - is that some dogs are bred to be great with kids. The golden retriever and yellow Labrador for example have unfortunately been labeled as this. All dogs are animals. All dogs are built for survival and self preservation.
All dogs are capable of biting when they are aggravated, assaulted or excited.
Dogs that are great with kids can be any canines that have been treated with the respect, time and training they deserve as intelligent and feeling beings. That's not to say that all dogs are good candidates for being with families that have children. We taught our son, Tyler, to respect our dogs as he'd respect any livestock. Our dogs are larger and heavier than he by about 40 lbs. It would make no sense to try to convince our five year old that these dogs are big fluffy harmless stuffed animals.