A book about kids thinking their neighbor is a witch

If i remember correctly, the books starts with a girl saying “NO!” (NON! in the french vesion that i read) to her brother or maybe her friend. Her and other kids spy on an old lady because they all think she’s a witch. The little girls ends up befriending the lady.
The parents of the girl met while being kids and spying on the lady. The we’re wearing halloween masks. The mother said that she took off her mask and kissed the dad and that’s how they got together.