Hi Jenna,
Guests with Advance Dining Reservations at a Disney Resort hotel are only permitted to park their car at that Resort hotel for a period of up to 3 hours.
As you will be visiting the Magic Kingdom for the day after your breakfast at the
Grand Floridan Cafe, you have 2 options:-
1/ Drive directly to Disney's Grand Floridian Resort & Spa for your breakfast and then once you have enjoyed your meal, move your car to the Transportation and Ticket Center (Magic Kingdom parking lot) before making your way to the Magic Kingdom park.
2/ Drive directly to the Transportation and Ticket Center and then from there, take the resort monorail to Disney's Grand Floridian Resort & Spa. The only catch with this option is that the resort monorail typically only starts operating an hour before the Magic Kingdom park is scheduled to open each day and with such an early Advance Dining Reservation, this may not be feasible.
Walking from the Transportation and Ticket Office to Disney's Grand Floridian Resort (via the walkways at Disney's Polynesian Village Resort) is possible, although you would need to allow approximately 30 minutes should you decide to do this.
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma