We were visiting PILs recently, and SIL was there with her toddler DC. I knew she used to smack her (much) older DC, but thought she had stopped this. Toddler was playing up a little, but nothing too bad, and SIL shouted and smacked them round the legs. Nobody knew where to look and nothing was said at the time. We have a small baby DC who will certainly not be smacked. I felt really uncomfortable at my DC witnessing that situation, baby did seem to realise something was wrong as was looking at me then back at SIL. We left soon after. I haven't had a chance to speak to DH about it yet, but I'm not sure how to bring it up or to explain how uncomfortable it made me feel. Without it sounding like I'm having a go at his family