I won't tell if you don't
A cheap promise you make to someone just to shut them up, even though you're going to tell everyone. It's like a fake friendship made by someone who's just trying to stay out of trouble.
'I won't tell if you don't,' he said, and then told the whole school.
'I won't tell if you don't,' she said, and then told her crush.
He said, 'I won't tell if you don't,' and then told his mom.