My husband and I have a placid GSD with an amazing temperament; he is well trained. He does play rough with our other dog but today my gorgeous nephews (aged 6&8) were over and the whole family was playing football on an oval when my GSD decided he also wanted to play. He mouthed my 8 yo nephew, almost in a tackle for the ball. There was no growling, he honestly thought he was playing and didn't do anything wrong. I was in total disbelief because I've never seen him ever behave roughly with humans. My nephew had marks from his teeth (no skin broke) and thankfully believes that he's football skills are so good that even the dog was tasked with trying to stop him but I'm very concerned and want to do the right thing as a responsible owner. We don't have kids so he doesn't have training on child friendly behaviour and I'm mortified! I'm a new GSD owner (he's 4 and he's our first GSD) and have sought lots of advice on training and raising him well and am tempted to have my mum babysit him when the kids come over so he's never in that situation again. Any advice on how/what to do is appreciated!!!