The sad thing is some folks think their dog is unfailingly protective of their children. In many cases, this is true. Yet given the right set of circumstances the dog will behave aggressively when the child does something that, in dog language, does not jive.
For example, we have 2 Yorkshire Terriers. When my 2 year old gets in either of the dog's face after chasing them around (under our watch, never to the extent of abuse), the dog will eventually growl and/or bare teeth. Of course, we correct both the kid and the dog immediately. However, when you're talking about a 50 - 80 pound animal and a toddler the possibility for grave injury is high. The possibility of grievous injury goes through the roof without supervision.
I've even seen some news stories (where the child is killed or maimed) when the dog is allowed free reign in an infant's room/sleeping area.
Here's my point: It's a dog people, not your child, act accordingly. This type of thing really makes me lose my temper.