You are right, every dog has the potential to do something harmful. However, most of the time when a dog snaps, including pits, is because the owner did not notice or ignored the signs when the dog began to show aggression. For instance, growling, nipping, trying to show dominance, etc. If your dog ever shows these signs or other potential threats, nip it in the bud, and you shouldn’t have aggression problems later. Caesar Milan (aka The Dog Whisperer) says this and he has owned a number of pits. He says that if people have a “beware of dog” sign on their property than they never properly trained their dog.
As far as a pit’s strength, yes they are strong. What might be worse in aggression situations is their ability to stay focussed on the task at hand. They are known for this. That is why when a pit attacks, people have trouble stopping them.
In terms of your freightened child, that needs to be dealt with quickly. The dog will probably learn that he is the dominant one. Dogs are intelligent and sense fear quickly. My daughter was scared for days when we first brought home our lab/pit, but she quickly got over it. She is now great with our dog. Have the dog lay down and be calm. Then have your child come over and pet the dog with you. Doing this should start to show your son or daughter that the there is nothing to fear, especially when the dog begins showing affection like licking. When your child gets used to the dog, have him or her get involved in the training and giving the dog commands.Also, have him or her feed the dog showing that the dog has to rely on the child to get what it needs. Have the dog sit and be calm before your child gives the food and gives the command to eat. In a little time these techniques this should help build his or her confidence because he or she will see the dog is submitting.
Be at ease, you have a combination of two very loyal dog breeds in one dog. Both are loyal, affectionate, and good with kids. I love that the pit in my dog makes her protective, but not violent. I have two young kids (seven and five), and our dog is great with them. She loves kids! However, I trained our dog early that the kids are her superior, especially when early on she growled at them. Good luck and congrats on the new addition to the family.