I have a small miniature pinscher that is showing signs of aggression towards strangers. People who are not afraid of him he dont attack them, first starting with barking then when they moving maybe he will start biting.. He dont like kids, he attack kids everytime.. when someone rang the door, he jump and bark... I said most of people who are lovely with dogs, not afraid have 80% friendship with my dog

I have never do something bad to my dog to have trauma or something like that..
I really dont know what to do..