Unfortunately, if she hit you and your siblings as children - she will carry that on down to her grandchildren. And she'll believe, 100%, every time that she is challenged about her attitude towards discipline and/or having hurt/frightened them, that she's right - and you, the actual parent, are wrong. 
My mother bit my son when he was around that age. He hated holding her hand (they've never liked one another), she kept tightening the grip, I was asking her to let go of him so that he could hold my hand, instead, her hackles went up and as she started to berate me for having the audacity to challenge her/remind her that she wasn't his mother, he did the only thing he could think of to make her let go of him. He sank his teeth into her. He'd never bitten before. Before I could grab him, however, she'd yanked his arm up and bitten him back. In public. On a "family day out". Not only did we go NC with her for a year, she had a visit from the police for assaulting my small child.
Your little boy is still too young to grasp when games end, I would have thought - especially if he was enjoying himself and everything was fun. He's also too young to understand cause and effect. Your mother, on the other hand, is not. She's supposed to be an adult, and she ought to have known better. But unfortunately - and I say this as someone who was once caught up in the FOG of a domineering mother, myself, who used assaults and abuse upon young children to get her own way - so should you. Compare your childhood to that of your son, and I'm pretty sure you'll admit that he has the better one. Please don't expose him to your mother's violence and bullying, sly, manipulative behaviour towards children too young to know any better, and certainly never leave him alone with her.
My son's 16 now and hasn't been left by himself with my mother since. Through what I now understand was bullying, immature nastiness towards him (I think she was getting some sort of kick out of him pleading for her to let him go, actually - having "power" over him), she's the one who is losing out. He'll only spend time with her, if I'm there... and I can't say that I blame him. At all.
I can count on the digits of one hand the amount of times my grandmother hurt me - and still have 4 left over (and she smacked my bottom when I was 7, because I'd done something that could have killed me, and which frightened her horribly). This meant I had a fantastic relationship with her, and would visit by myself, call her for chats as I grew up (I was in my mid-20s when she died), had little "in" jokes with her, and miss her to this day. That, surely, is what a relationship between a grandparent and their grandchild(ren) is meant to be like... not one couched in fear and resentment.
because I know it's not going to be easy, but also a suggestion that you look into therapy in order to try and untangle those chains that bind you to the belief she'd never dream of treating your child, the way in which she treated you. 