As the mum of a threenager I found this to be quite an apt phrase to remember.
We've all seen the 'keep calm' posters in many forms but a phase is something well worth reminding yourself when your child is lying flat out on the supermarket floor because you have refused to let them have a magazine/chocolate/crisps. Or refusing to wear their coat/get into the car/walk/sit in the buggy* delete as appropriate.
You must remember it is just a phase and it will pass, who cares if the world and his wife are watching you giving you that knowing 'if been there' wink.
So next time you are being driven to distraction by your child. It's just a phase. Keep calm it will soon be bedtime!