Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
Many dogs have territory problems with babies and especially new babies. A muzzle will not help in this situation because there is always a chance that the muzzle is not on or that this still will frighten the children.
This is a common problem with some male dogs. The best thing to try is an ultrasonic collar. Use it when you are present and it sends a high pitched signal that the dog hates and will discourage this behavior. I have also had very good results with the collar combined with a drug called Reconcile. It is a prescription medication that acts as a calming agent and will, along with the collar, retrain your dog so the aggression is minimized. I know this sounds severe but this is a very serious situation and you do not want to put your family or others at risk. Call your vet about this product. The package insert will have behavioral modification techniques that will help.
It is also recommended that you have your vet examine this dog to be sure there is not a medical reason for this change in behavior. Take this dog back to Obedience School so you will have better control over Humphrey so your grand children will be safe. It is not worth the risk of someone getting hurt.
Here is a website that has more information that might be useful for you:
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, ***** *****