Best advice is animal control and avoidance what ever that takes!
That dog sounds like bad news! K9 unit sounds like a good idea, they might be able to at least scare the crap out of the owner?
Other advice is to always remain calm and face the dog down, I'm willing to "engage" if it gets down to it. Arming the kid with bear spray would be more practical, turning and running if they are close to safety could work but your not going to out run a dog for any distance!
Putting "heat" on the owner is the best advice!!!!