Not a daft question at all!
Yes, that's the right number. Start calling a couple of minutes before 12 as they often open early - these things go so quick it IS worth the effort to do this.
If it's too early, you'll get a message saying the lines are closed - if you get that, hang up straight away and hit redial. If it's open, you'll be asked by one of those automated messages whether you want Dining or Victoria and Alberts (dining!) you now get asked if you live in Florida and whether it's your first trip, or atleast they did the last time I rant (hit 1 for yes sort of thing).
Once you've done that, the line will ring again as they put you thru to dining - as soon as they answer say "Hi, Cinderellas breakfast for X people, any time" They should be ready for you anyhow as that's what everyone is ringing for!
Last of all, if you are not successful, don't give up...try ringing again about an hour or two later. Lots of people get all their friends and family to call so it always gets some double bookings, if you are lucky, by then some will have cancelled the extra's and you might pick up a 'stray'
Good luck!!!