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")
Now, as far as I'm aware, Hogwarts Express always departs from Platform Nine and Three-Quarters.
And Molly have been there innumerable times, both as a student, and many many years prior to Book 1 with older Weasley kids going to school.
Hell, she even knows easily how to get onto a platform:
'Not to worry,' she said. 'All you have to do is walk straight at the barrier between platforms nine and ten. Don't stop and don't be scared you'll crash into it, that's very important. Best do it at a bit of a run if you're nervous. Go on, go now before Ron.'
So why did she not know/remember the platform #?