It wasn't a lightning bolt of revelation, nor even something that slowly crept up on her. It just is! One minute she did believe in the roly-poly bearded fella; the next she didn't.
Last year, she'd lock herself in her room for hours on end, scribbling her long letter to the North Pole.
This year, she sarcastically raises her eyebrows whenever Santa is mentioned.
Such as last month, when I asked her what she wanted for Christmas.
After 10 minutes of reeling off every present known to kid-kind, I said: "Don't tell me – tell Santa. Write him a letter."
"Yeah, right," she replied, and smirked. And walked away, laughing.
But she still keeps up the pretence of believing, for mine, her mum and her dad's sakes (because we're not quite ready to wave goodbye to those 'baby years' even now!) but mainly for her younger brothers.
They do believe in Santa – with a passion that is so sweet it' could rot your teeth.
The eight-year-old especially. Each day, he opens a door on his advent calendar and checks the Norad website for the countdown to when Santa will set off on his present-delivering journey. When he got to sit on the great man's knee at the school Christmas Fair two weeks ago I thought he was going to pass out with awe.
For him, the existence of Santa is an absolute indisputable cast-iron fact. Which the other day presented a bit of a dilemma.
It happened when I found a note to Santa written in my son's scrawly handwriting. which read: "Dear Santa, I believe in you. How do you manage to deliver presents to all the children all over the world? I have already sent you my list and I have been a good boy. Love Tom xxx".
That should have been that, but then the next day, I found my stepdaughter writing a reply – as if from Father Christmas.
It read: "Dear Tom, Thank you for your letter. I can deliver presents to everyone because of the time difference. If you stay being a good boy you will get all the presents you want."
OK, big deal. So what's the dilemma? Well, it was two-fold:
1) I didn't want my son to think Santa had written to him when he hadn't – because I thought he would be crushed if he discovered the truth i.e. that it was his sister playing him along; and
2) He isn't going to get all the presents on his list because neither I, nor Santa, are Russian billionaires and haven't won the Lottery.
So what's a dad to do? Ensure the lad never saw 'Santa's reply' or let him read it and play along with the fantasy and deal with the consequences as and when they unfolded? I decided on the latter and crossed my fingers.
Later that evening, Tom found 'Santa's reply' under his pillow and rushed into his sister's bedroom like a kid whose Christmases had all come at once.
"Daisy, Daisy," he yelled. "Santa's replied. He says I'm going to get all my presents if I'm good."
I wasn't sure what would happen next, but I half-expected Daisy to lift the wool from his eyes and shatter all his dreams by revealing the truth of the letter.
But instead, she studied the letter and declared it authentic, saying she'd had a letter like that when she was Tom's age.
Then my son came into the kitchen to see me.
"Have you seen my letter from Santa, dad?" he asked.
"Wow, son, that's amazing," I replied.
And then he floored me.
"Don't worry, dad, you don't have to pretend. I know Daisy wrote it. She was just being kind," he smiled.
"Besides, that's not Santa's handwriting!"