Generally, two year olds understand what “NO!” means. You don’t have to read them a damn manual. They aren’t idiots. Most can be taught NOT to do something.
Sure two year olds know what *No* means and they know how to use it. So when a two year old decides to assert his little independence, he gets mauled by a dog.
It’s not worth the risk that it will happen again. That dog does not have a good temperament .