Because here is a dog who she knows has growled at children before (if she isn't making that up) and she left a child alone with the dog. Something that shouldn't be done even with the most trusted family pet.
I believe a dangerous dog order is the correct course of action in this instance. It will mean the owners have to have a specially built child proof enclosure, the dog will have to be muzzled whenever it is not in this enclosure and they will have to display signage denoting a" dangerous dog" on premises. These are all precautions that will prevent any further incidence with this dog and hopefully cause enough financial loss (on building enclosure etc) that they will reconsidered ever putting their dog or another dog in that position again.
Hoping your son doesn't need surgery and hoping he is feeling ok today. How is he with your dogs at the moment?