"Oh dear, you are being a bit naughty/silly aren't you? D'you know what, i think it's because you are really tired."
I also thought this was to the OP!!
BUT
That was a naughty/silly thing to do is a far better way of putting it.
Have to be careful not to create a self fulfilling prophecy with children, I'm called naughty therefore I must be, so I will be etc..