Disclaimer: J.K. Rowling owns Hogwarts Express and all its passengers and all its stops.
Parts of this story are quoted from Harry Potter and the Philosopher's Stone.
1.
Nice September day...
King Cross station...
A redhead family is walking, apparently looking for their train.
"Mum, can I go too" pleaded a little girl. "With Fabian and Gideon, please?"
"Not yet, dear." said the mother, and then, after looking at girl's disappointed face, she added:
"By the way, anybody remembers the platform number?"
"Nine and three-quarters!" proudly declared the girl.
"Thank you, Molly" said the mother.
"Thank you, Molly, thank you" added the brothers, grinning.
2.
Another year, another first of September...
Heart hammering, Harry pushed his trolley after them.
...
"Now, what's the platform number?" said the boys' mother.
"Nine and three-quarters!" piped a small girl, also red-headed, who was holding her hand.
"Mum, can't I go..."
"You're not old enough, Ginny, now be quiet. All right, Percy, you go first."
3.
26 years later...
Same train station...
"Lily, let me ask you, do you remember what the platform number is for Hogwarts Express?"
"Mum, you do know the platform number; yes, you do."
"Well, Lily ... but it's a tradition!"
"When I was a little girl, like you, accompanying my brothers to Hogwarts - Grandma Molly asked me that."
"And when she was young - her mother asked her that ..."
"Stupid tradition" said the girl stubbornly, "And, dad, why are smiling so?"
Harry Potter squatted down by his daughter and quietly told her:
"Lily Luna Potter, do you know what were the very first words I heard your mother say?"
"It was that number ..."
"Tradition, you know ... just say it".
"Oh, well", sigh.
"OK", sigh.
"Quarter to nine! Happy now?"