In the first Harry Potter book, Molly is seen asking her children "what platform number is it again?"
Heart hammering, Harry pushed his trolley after them. They stopped and so did he, just near enough to hear what they were saying. '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.' (Harry Potter and the Philosopher's Stone, Chapter Six, "The Journey from Platform Nine and Three-Quarters")
Why didn't she remember on her own? She would have been bringing kids there for the past 10 or so years, not to mention her own years at Hogwarts.