MIL babysitting woes?

My MIL had 8 kids, & has grandkids in middle school. I had a preemie son, who's now 3 & still very tiny, & she has never been allowed to babysit. My husband, nor I, trust her with him. She completely ignored our wishes raising him, such as not being around him with a cold, always washing her hands, and general safety while he was a preemie. She's based him off her other grandkids & how she's raised her 8 over 50 yrs ago, & doesn't see a problem with doing things her way, even if it goes against our wishes, & flat out said she'd do what she wants when alone with him . She's placed him on toys meant for older children at 6 months old, leaving the room for 'something important'. She never watches what he's doing when she holds him, or in general, and curses. She's close to 70, & can't keep up with a toddler as it is. She's once again pushing the issue wanting to babysit. What wording can I choose to NICELY say we don't trust you or want you babysitting without starting WWIII?