Start by determining a visit schedule that's as close as you can get (without losing your mind) to a complaint-proof frequency.
Then, choose terms for the visits that help you avoid predictable conflicts. For example, if she's always on your case about the way you run your household, then visit on her turf; if she refuses to childproof because "you should teach your 2-year-old not to pick up knives left on the floor," then visits are mostly at your house or kid-friendly venues, like parks or children's museums.
Also, have distracting activities planned, since busy people are much less obnoxious than bored ones (this goes equally for kids, needy adults, eye-rolling adults, and fed-up adults who bottle up their frustration till they snap).
Combine these steps with the basics of keeping visits short, avoiding contentious topics, and acting as a gentle buffer between Grandma and grandkids, and it might be possible to keep her in your kids' lives without it becoming a drain.
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