I had a boxer when I was growing up. She never growled, bared her teeth, etc. She had pet rats, kittens, parrots, kids, etc. crawl over her and never reacted. She had her ears pulled, was poked and prodded, etc. and never reacted. She took the Guide Dogs for the Blind's guide dog obstacle course and got a perfect score (time after time much to the chagrin of the guide dog puppy handlers). She NEVER behaved in any way, shape or form aggressively, etc.
UNTIL one day we were at the park and some 20-something guys came up to us with their lab. They acted very aggressively towards us and threatened to have their dog attack us.
Our Siberian Husky ignored them and the dog. Our Belgian Shepherd ignored them and the dog. The boxer got up, stood in front of us, raised her hackles and growled at the dog and guys.
Any dog is capable of anything given the right set of circumstances. In addition, a dog who is being protective of a baby/toddler is very likely to try to grab the child by the scruff of the neck to pull it to safety. Unfortunately, children don't have a "scruff" to grab so the dog ends up hurting the child that it was trying to protect.