Welcome back, Candice!
My family and I have had the opportunity to enjoy Afternoon Tea at the Garden View Tea Room at Disney's Grand Floridian Resort & Spa and it's such a unique experience. You're really going to enjoy it!
When needing to transfer between Disney's Grand Floridian Resort & Spa and Magic Kingdom Park, you have two options:
Disney Monorail Transportation: The Monorail platform is located on the second floor of the Grand Lobby and is how I suggest getting back to the Magic Kingdom after your tea; Magic Kingdom is the very first stop. Conversely, it's not the fastest way to get to your reservation as the Grand Floridian is the last stop on the Monorail Resort Loop.
Disney Water Transportation: There is a boat launch from the Magic Kingdom to Disney's Grand Floridian Resort & Spa with continuing service to Disney's Polynesian Village Resort. This is how I recommend getting to the Grand Floridian, as it is the first stop when utilizing the boat transportation option.
There is not a footpath connecting the resort to the Magic Kingdom. Though it does seem like there would be, there is not. Fun Fact: There is a well-hidden canal that connects Seven Seas Lagoon to the water attractions (like Jungle Cruise) inside Magic Kingdom. They also use this canal to store the floats used in the Electric Water Pageant. You may be able to see them on the Magic Kingdom side of the Monorail as you travel back.
I hope this information is useful in your planning, Candice! Please let me know if you have any additional questions. I am here to help!
Faith