There's no evidence showing that big dogs are any more likely to harm children than small dogs. Chances are, you already know some very sweet big dogs, and if you don't, the ASPCA or your local shelter would be happy to introduce you to one.
There's been some recent debate about the inherent natures of pit bulls in particular, but again, there's no evidence to show that pit bulls are more likely to cause harm to humans than any other breed. A dog's -- any dog's -- behavior is a function of many factors, including breeding, socialization, training, environment, and treatment by owners.
Myth: Getting animals from breeders is safer because the breeders know the animal's bloodline and family history.
First know that, as a result of their breeding, purebred dogs very often have genetic disorders and medical issue predispositions -- certainly no less often than shelter dogs. Also, while bloodlines and histories are useful tools to assess an animal's value, they are limited in terms of predicting behavior. On the other hand, shelters are motivated to save lives and make strong matches. Some use science and sophisticated tools to appropriately pair up animals and owners, and are happy to share everything they know about each animal.