Children's book where nice mom/grandma goes upstairs to change into evil witch outfit to scare naughty children into behaving

I just woke from a nightmare about a non-existent scary book character, and it made me recall this book that used to scare me as a kid, but I can't find it anywhere when I google it.

I think it was about a grandma who is visiting to babysit the kids. She's very nice (blonde? smiling), but the children are misbehaving and won't listen to her, so she sneaks off upstairs and dresses up in a scary black outfit (witch outfit?) and comes down to scare the kids every time they are misbehaving. Eventually they stop misbehaving because they are scared of this evil lady.

I don't think it was meant to be a particularly scary book, but I remember being scared of the illustrations of the 'evil' lady.