Newfoundland Dog Issue?

You need to take her to a vet FIRST, before the behaviorist. Be sure you tell the vet what happened. You need a physical exam to determine if the dog is in any type of pain (like hip dysplasia) which is EXTREMELY common in your breed. Petting her anywhere near the hips could cause pain & she might bite/snap as a warning. She could have an ear infection (so bad the tonsils may even be swollen) a tooth infection, etc. (She is the right age to be cutting molars and a yanked-on collar might really make them hurt). You might also ask to have a blood panel run to rule out hypothyroidism which can make some affected dogs suddenly aggressive. If the vet rules out any physical cause, then ask for referral to a behaviorist.

Was your dog evaluated by any one reputable or knowledgeable, before you adopted her? It is really best to adopt from a reputable rescue group or shelter that does behavioral evaluations before adopting a dog out... especially when there are children involved. Not all dogs LIKE children, regardless of the breed's general charcateristics. What made you take a dog that had been abused into your family (esp since you had children who are more vunerable) & assume the abuse would have no impact or issues?

Have you taken this dog to any formal obedience classes? (That is also an absolute requirement in most cases of reputable dog adoption) esp when there are children in the home and with GIANT dog because you need control & good manners. (The dog needs to thoroughly understand what is expected of it and to be able to mind commands given by anyone in the home, including kids.) Had you already done this training, you would have a far better READ & understanding of the dog, in a variety of (potentially stressful) situations.

Have you considered being around yelling or high-pitched (excited ) kids, esp a greater than normal number, may have overwhelmed/frightened the dog? What if one of them moved in a way that the former abuser did just before s/he abused the dog? Previously owned dogs, often have emotional "baggage" and need to be worked with.

She may not have suddenly flipped out. She may have been showing signs for a while that she was uncomfortable with the situation (lots of licking lips, turning her head away, etc) and when no one responded to her body language, she felt like she had to escalate her message. She likely growled or snapped in the air. On purpose. if she had wanted to cause damage she would have. You'll notice that this escalation in her message made it come across.

Or possibly she really did just snap suddenly without warning. This would be more common among dogs that have been physically punished for growling. It takes away an intermediate step of escalation (between body language and biting) that is very noticeable.

You should do some research into fearful dogs and body language signals. If you are aware how your dog is feeling (hint - if there are a bunch of unfamiliar kids petting her, she's probably not happy no matter what her tail is doing), you will be more able to predict her behavior, then remove her from situations that are so upsetting.

A behaviorist may be able to help you. When choosing a behaviorist, make sure they do not use the idea of dominance or submission to explain their process. Applying these (scientifically unsound, but sometimes effective on some dogs) principles to fearful dogs often makes the problem worse.

EDIT - and about your kids, teach them about how to tell if the dog wants to be left alone. And don't leave any children unsupervised with her.

I agree, you need a behaviorist. It sounds like she has fear aggression.

I agree - get in a behaviorist and get the dog checked out.

Get a Behaviorist NOW.