This is good for any new parent to know!

ipse dixit (IP-see DIK-sit) noun

An assertion without supporting proof.

[From Latin, literally, he himself said it.]

The term ipse dixit is the Latin equivalent of the Greek autos epha, referring to Pythagoras, as in, The master (Pythagoras) said it so it must be true and no proof is needed. In our modern world, this has many forms:

Child: Why do I have to go to bed at eight every day?
Parent: Because I said it.

Employee: Why do we have to do this project if it's going to be scrapped anyway?
Boss: Because I said it.