Grandparent

Hi iv been with my partner for 10 year she's 38 I'm 44 we have 2 kids together boy of 6 and a girl at 4 the trouble is....everytime my partner's mum comes round I don't like the way she is with the kids.....as soon as she walks through the door if there playing she tells them to sit down and not move....if the 6 yr old wants the toilet she tells him to go and come straight back down....as for discipline both myself and partner does that we don't believe in hitting children...but when my partner tells her mum the kids have misbehaved her answer is give them a good smack....Iv tried talking to my partner and telling her I think it's wrong her mum isn't the kids parents she can discipline for example if she's babysitting them and we ain't there....just like school can give detentions etc when the kids are in there care but my partner thinks I'm being out of line...iv tried talking to my partners mum and told her I don't like the way she's trying to control the kids and all I get is it won't do them any harm it never did her any harm....I'm getting to the end of my tether I dnt want to go down the route of stopping her seeing her grandkids I'd be greatful for any advice....sorry this post is so long