Howdy Shannon!!
I am so excited for your son, husband and dad!
Disney's The Magic Behind Our Steam Trains Tour sounds amazing!! I'm going to send my son, hubby and dad on the tour as well. My son is only 4 so we have a few years before he will be able to go, but this tour is definitely going on the must do list! THANK YOU!!
I checked with a Cast Member and found out there are a couple of options for getting from the Resort to Magic Kingdom. The monorail will start at 7:00 AM so they should be able to get to the front of Magic Kingdom in time for the tour. (As FYI, the first boat typically starts around 8:30 AM.) Further, the buses go to and from Disney's Grand Floridian Resort and Spa to Magic Kingdom Park only when the monorail is down.
If you are still nervous about them making it to Magic Kingdom in time, they might also consider taking a taxi from the Resort to Magic Kingdom. Once you arrive at the Grand Floridian, I would suggest that you chat with a Cast Member at the Concierge Desk and ask them for their advice. I want to insure they make it to the tour on time and the Cast Member will know more about the current schedules.
We will be staying at The Villas at Disney's Grand Floridian Resort & Spa in a few months as well and I cannot wait! It's going to be amazing!!
Thanks again for your question and please let me know if you need any further information. Have a magical day!