Hi Celia:
You've picked a great restaurant and a great location,
1900 Park Fare at the
Grand Floridian Resort and Spa, to enjoy your daughter's third birthday.
The Grand Floridian is very close to the Magic Kingdom. The "best/quickest" way to get to the Grand Floridian is to hop on the monorail, located right outside the Magic Kingdom. The resort-line monorail makes a continuous loop, and in your case, it will "start" at the Magic Kingdom and "end" at the Grand Floridian, making stops at the Ticket and Transportation Center, the Polynesian, and The Contemporary along the way. The monorail ride will take approximately 15 minutes, and I would factor in between 5-10 minutes extra of waiting time for the monorail. I wouldn't be afraid of arriving at the Grand Floridian early and checking out the beautiful lobby or strolling through the Resort's shops.
Another equally "best/quickest" way to travel from the Magic Kingdom to the Grand Floridian is by boat, which can be found at the boat launch right outside of the Magic Kingdom.
After you finish at the Grand Floridian, simply hop on the monorail or boat to return to the Magic Kingdom for your dinner. Remember to check in at City Hall at the Magic Kingdom and pick up a birthday button. Many Cast Members will go out of their way to wish your daughter a Happy Birthday or greet her with a kind gesture to make the day more memorable.
Enjoy your vacation.