Definately both at fault here.
never assume anything positive about dogs you don't know.
Unfortunately I've met a huge number of sometimes rude but often downright aggressive dogs and their idiot 'he's only playing' owners...
Your dog should not be racing off at great distance to see other dogs.
He ran so far ahead you didn't even know the breed at first, that is ridiculously and dangerously far away because firstly such behaviour is immensely rude and threatening, you will never see dogs with good social skills race like a bloody loon to see other dogs miles away and secondly, being a very small dog, a big and highly prey driven dog could potentially mistake him for prey with disastrous consequences.
The Dobermanns should have recalled. Definately.
Your dog was not 'attacked'.
Either the Dobermann has been a bit of a bully and was deliberately intimidating him or the Dobermann was trying to play (roughly) which frightened your dog.
Believe me, if a Dobermann had attacked your dog for real, as in bit and made contact, you would not have a jack Russell left.
If not killed outright he would have been left with very noticeable and serious injury.
They are so fast aswell, don't kid yourself that it was your intervention that saved him.
If the Dobermanns intention was to injure him, he would be injured.
As for what if it was a child 
There is NO correlation between dog on dog aggression and human aggression.
Absolutely none.