We love CP breakfast! I'd say it takes about an hour. We usually get an 8:00 ADR and we're heading over to the rides at 9:00 or a few minutes after.
If you're done with breakfast at 9:30, you won't have any trouble grabbing a FP for a ride, but Dumbo will probably have a bit of a wait by then. Although in Sept the wait might not be too bad - I haven't been at that time of year in a very long time. If it's a day where there is morning EMH, you'll have more crowds by 9:30, but if it's a regular 9:00 opening, you should be fine.
Also, you might want to show up a little bit early for your ADR. You might be able to be seated earlier than 8:30. And, if not, you'd have some great opportunities for uncrowded pictures of the castle and Main St. There is usually a Photopass photographer at the end of Main St, so you could get some nice family photos.