Hey Travis!
Welcome to the Disney Parks Mom Panel! Thank you for bringing us your question. You have a picked a fabulous time of year to visit Walt Disney World. Several great events take place during the month of March including
Epcot's International Flower & Garden Festival,
Atlanta Braves Spring Training and
St. Patrick's Day at Downtown Disney.
The trip from the Magic Kingdom to the Disney's Grand Floridian Resort & Spa via the resort Monorail or boat launch will take approximately 15 minutes. However, I suggest allowing for 30-45 minutes just in case there are longer than average wait times. If you happen to arrive a little early you can either try to be seated right away or explore this magnificent resort.
The return trip to the Magic Kingdom will only take 10 minutes or so via Monorail as this is the last stop before the Magic Kingdom. As long as your dinner reservation is no later than 7:30 p.m., assuming that Wishes Nighttime Spectacular is at 10:00 p.m., you should be fine. This will give you plenty of time to get back to the Magic Kingdom and find a great sport to view the fireworks and maybe even stop by one of the shops on Main Street, U.S.A and get a little dessert.
Remember if you are planning on visiting the Magic Kingdom during the day then leaving to go to dinner you will need to have purchased the park hopper option on your park tickets so you can return to the park the same day.
I hope this helps. If you have any other questions please come back and see me.
Wishing you a magical Disney vacation and pixie-dusted day!
Sue N., IN