Kids have no filter. It's one of their most precious and entertaining qualities. But it's not always an ideal quality for their next-of-kin, especially if they're trying to keep secrets (and most families are). When kids find out about those secrets, they will likely tell anyone they come in contact with. Including their teachers. This is yet another reason for parents to be extra-nice to their kids' teachers during parent-teacher conferences—because you never know what they might know.
Someone asked teachers of Reddit: "what are some family secrets you know about people because their kids told you?"
1.) From Snoo-32987:
Child, “daddy dropped me off today!”
Me, “Oh, fun! Where’s mom?” (Mom always dropped off)
Child, “mommy’s getting a camera in her butt!!!”