Here are my favourite bits of a Reddit thread about the lies parents tell their children (along with a couple of my own):-

If you don’t learn how to read, your voice disappears.

If you pull a face, the wind might change and you’ll be stuck with it.

You can’t eat that, you are allergic to sugar.

If you press a certain button on the remote control the TV will explode. 

When the ice cream van rings its bell, it’s to warn everyone they are out of ice cream.

Eat your crusts and your hair will be lovely and curly.

The Internet is switched off at 6pm.

The plane will turn around if you misbehave,

If you don’t sit still the barber will cut your ear off.

The sweets on the checkout aisle aren’t for sale.

Rice cakes are real cookies.

Eat your carrots to be able to see in the dark.

The TV only works when it rains.

There are no replacement batteries for this toy.

Eat spinach to be strong like Popeye.

When you lie, a red spot appears on your forehead.

Pulp Fiction is a documentary about oranges.

If you burp, fart and sneeze at the same time, you will turn inside out. (This one is for +Xeb eke ).
Santa has asked for Doritos and beer instead of milk and cookies.

If you keep eating vegetables they will start to taste like sweets.

The door is locked because mummy is helping daddy hang a picture behind the door and we don’t want you to open it and hit us.