Well, the growl is actually good. A growl is a warning--please stop what you are doing because it makes me uncomfortable. However, whatever has convinced the dog that she has the right to tell the kids off needs to stop. And yes, this is absolutely solveable with training, at least for most dogs.
If you have the money, I'd schedule an appointment with a behaviorist asap. There's lots of advice I could give you, but without seeing the dog I could be reading the situation totally wrong. There's a big difference between a dog who nips because she's ticked off and a dog who nips because she feels threatened. That's why whenever a dog has actually touched a human with its mouth, I advise that a professional see the dog.