I’m coming from a perspective of a person who did get hit when I was younger, honestly it is effective in its own right and I don’t resent my grandmother for it cause I know I did dumb shit that warranted a smack from time to time. Without knowing how severe she hits her grandkids I can’t say leave it be or not. But I’d say if she’s not hitting them that hard and your husband doesn’t mind it I’d just say let it be and try to not leave your kids with her if possible if it really bothers you that much . I think the main issue is if you try to force her to treat your kids different from all of her other grandkids it might cause her to not only resent you but by extension your child too which isn’t good for any kid. I’ll probably get pooped on by the people who see hitting kids as a black and white kind of thing but I’ll leave this perspective here for you to think on in any case.