LOL, have you ever met a Cairn? Most of them are game to take on whatever comes their way.
Oh no, I don't doubt that a cairn could hurt a 6 month GSD. Like I said, we have a westie in the family and he only backs down from my GSD when the GSD is really really pissed and the westie can tell he's going to do something.
I just don't think its physically possible for a cairn to punch a GSD in the face...he could lunge and bite but not punch (unless the GSD is laying down).
I read it as the cairn came over to the group, might've played a little rough, and the GSD pup (like GSD pups tend to do) punched the cairn in the face, the cairn didn't take so lightly to it and decided to tell the 6 month old who's boss. In this situation...the GSD is at fault, if I'm reading it wrong then hopefully the OP can tell us exactly what happened.
I just hate blanket statements like 99% of dogs in that park are great, and this is the only one that is terrible. Its the only one that isn't trained, ect, ect. I've gone to dog parks all my life and 9/10 dogs aren't trained well enough (for my tastes) when it comes to recalls and call offs. I'll admit that my dog isn't aggressive but he does like to throw his weight around...I have to make sure he's not trying to show his dominance to the wrong dog because that dog might react and then my dog won't back down. I've seen my dog at least "try" to start plenty of fights.
When it comes to dogs and their language, it usually takes someone very knowledgeable and very attentive to see what is actually happening in a dog pack. And sadly, 99% of people that go to dog parks aren't those.