My mother spanked my son

Tl;dr: my mom spanked lightly my toddler even though I am against it. How should I address it?We are visiting my parents for the holidays. Earlier today, while I was chatting with my brother on the other side of the living room, my son (almost 3), who was playing closer to my mom, saw a cup of water laying around and decided to push it on the floor. My mom started yelling and stood up, grabbed him by the arms and hit him, lightly, over the diaper. I asked what happened, and she screamed at me that my son always ruins things willingly, and he needs more discipline, and that now I had to clean up the mess. I told her that she cannot hit him, she replied that a little spanking has never traumatized anyone. Then I went to grab a washing cloth, while my husband grabbed the child and started talking to him. I was pretty upset, but both my husband and child seemed completely clam about all. This was almost at bedtime, so I went to put my son to bed and won't get a chance to talk to my mom until tomorrow.I consider my mom a good and loving parent, but she was rather angry when I was growing up, and hit me regularly for almost as long as I was living with her. Usually it was light and didn't hurt much, but a few times she lost control and it got quite scary. I remember feeling like I was always the bad kid and everything was my fault, so what she said about my son always ruining things really rubbed me the wrong way.I, on the other hand, am extremely soft as a parent, a lot of people consider me too weak, and maybe they are not totally wrong. I never get really mad at my son, it just doesn't happen to me. I talk to him about what is right and wrong, but I almost never punish him or physically restrain him, and I would never hit or spank him. I sometimes do think that he does need more discipline, but I hope that he will eventually understand what we tell him, and reach an age where he gets that some actions can make people sad.Still, I hate what my mom did, and I don't think it's her place to discipline him when they meet only a few times a year. I thought that my mom would be different as a grandmother, but I'm worrying about it now. Tonight wasn't serious, and luckily nobody got hurt or traumatized, but what if she loses control completely another time? What if it has already happened, but I wasn't there to see it? I just don't want her to allow herself to do it to my child. How can I talk to her? I'm afraid she won't listen. via /r/Parenting https://ift.tt/2QdqO1g