It's hard. But it's true all dogs are capable of aggression. We had chihuahuas who were very gentle and peaceful, not the stereotype of yapping little things, but disturb their sleep or get in the way of their dinner and they have lightning reflexes and sharp teeth.

I think you might need professional help on this one. I'm not sure what you should do, but you should probably separate the dog and the baby at all times, children can accidentally hurt dogs and the resulting instinct of a pitbull would not be good.
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