German Shepherds play loud and rough. It looks and sounds like they are killing each other. They are misunderstood by other breeds and that is most likely where a fight might come into play. My female GSD is more vocal then the males. My males slam each other into the fence. This kind of play can freak out people that don't know the breed and its a big reason they don't always do well in dog park or dog daycare settings. I've never considered not letting mine play like that, it's who they are, so I'm not sure if you can train who they are out of them. They aren't being aggressive at all. Not sure if this is what is going on with yours. If it is, then doggie daycare person would be mortified by mine and think they were killers.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD