Ah yes, I am surrounded by people like that, both friends and relatives.
I've had to roll with it. Occupy the kids. Maybe Grandma comes, maybe she won't come, who knows when she'll come. Here's something to do in the meantime. And if she arrives when you've gone out, well, it's HER fault, isn't it? And when she takes them, and you are free to do whatever, well, maybe you go out with friends and not look too hard at the clock...