Its a tough call, but when it comes down to it. Its a threat to your child, pet, and peace of mind. Even if you took action and shot the dog (not saying thats the best course of action), then the owner mysteriously showed up and tried to sue you. Its negligence on their part for not having their animal on the leash and in addition you tried other methods. Such as animal control, which is pitiful that they wouldn't do their job because they were 30 min to getting off.
I had a similar experience with a raccoon coming after my dog, a 20lb bichon. I immediantly picked up my dog and literally punted the raccoon with all my might about 20-30ft. I put my dog inside and checked to see if the raccoon was alive. I'll save you the gruesome details, but it was dead. Not that the raccoon was someones pet, but sometimes you got to take action in order to protect yourself and pets.