Hi Jay,
Thank you so much for visiting the Disney Parks Moms Panel with your question. I really enjoy breakfast at the
Grand Floridian Cafe. It is an elegant restaurant with delicious food!
I believe that the answer to your question is "yes," assuming two qualifications: one, that you have a reservation for breakfast and two, that you use valet, not self-park. If you self-park, you will be limited in the amount of time that you can leave your car in the parking lot at Disney's Grand Floridian Resort & Spa.
I called and spoke with a cast member at Disney's Grand Floridian Resort & Spa who explained the above to me. Also, when I visited last December, I experienced this at another resort. We valet parked our car at Disney's Yacht Club Resort, and the cast member there let us know that we were free to leave our car there all day based on our lunch reservation at Captain's Grille. I do suggest that you double check your plan with the cast member who valet parks your car on the day that you do this just to be sure that it is still allowed when you visit as policies can change.
I hope that this information is helpful for you, Jay! Have a wonderful time planning your vacation, and please do not hesitate to ask if you have any further questions.