What (someone) doesn't know won't hurt them American British informal
Being ignored or uninformed about something frees one from having to worry about, feel guilt for, or be unhappy about it
Jane: "What are you doing with your car?" Marry: "Don't try to ask me. What you don't know won't hurt you."
Linda: "My mom will criticize me if she knows my bad academic results this time" Jenny: "Definitely, so It's better to conceal this, what Mom doesn't know won't hurt her."