How to say no to grandparents

We’re a non - sleepover household, my children won’t be doing sleepovers for personal reasons… except for my mum’s on occasion if i need it. Works out less than once a month, maybe once every 2/3 months. My mum is the only person i remotely trust with my children. She just knows how I wish to parent and things I would or wouldn’t be comfortable with. We’re a family that’s dealt with abuse, hence my strictness. No one else has unsupervised access to my children. So for the other side of grandparents we go round for tea once a week so they get plenty of chance to be with my children regularly without having to be on their own. Partner’s mum has offered to have them in the past and I’ve said I know the offers there. She’s now pushing for sleepovers. How do I kindly say no that’s not something that’s happening? I’ve thought about saying “I know you’ve offered but we’ll ask you if that’s something we’re going to start doing” I like her, I just don’t want to pass over responsibility of my children. I don’t want to upset her but it’s a clear boundary I want to uphold and I need her to accept it.