Early signs of a dog thinking it's "alpha" over a child is mounting from the rear or deep, almost inaudible growling, baring teeth, etc. That crap needs to be corrected immediately. ("Mounting", even a female dog will do this, and neutered males will too. It's not always because it's horny. It's a dominance thing.)
and no matter how well you establish dominance, a true alpha dog will still test you from time to time.