Sometimes people just get careless and forget that they don't know the dog, even if they "know better." I've had new dogs that seemed fine and in the moment, I forgot that they really weren't and I knew that, moved too fast and been offered teeth. It happens. It doesn't make them killers.
Definitely this was a BIG handling error with a new dog. A dog that has just been moved from one home to another or a home to a "shelter" to a home, needs careful handling until trust has developed with the new humans. Under certain circumstances you might have to do things that you haven't really built the trust with that dog to do easily, but in those circumstances you would proceed knowing that you're provoking the dog and take appropriate precautions.
The girls should be talked to and I think a study of dog behavior or maybe a meeting with a trainer or behaviorist would be helpful both with this dog and in their lives moving forward. They may know a lot by instinct, but they would be well served to learn more about body language and safety. Learning that stuff will make it far less likely that they are going to have an issue with this little girl or any other dog, for that matter.