I have changed my name for this for personal reasons.
Dp and I are friends with another couple. We were quite close to them. The problem is the way he treats his kids. They have two boys who I must say are very naughty and badly behaved BUT that's life and didn't bother us. But the dad hits them - a lot and in front of us and our daughters. He twists their arms, smacks them round the head and dp has seen him punch one of them.
He is such a nice guy most of the time but obviously we find this behaviour really upsetting. We don't know what to do as it has got to the stage where we avoid seeing him with his children. Dp doesn't want to say anything as he thinks it is none of our business. It is a real dilemma.