What an awesome vacation your in-laws have planned for your girls, Liz. They truly are very generous to take them on this magical vacation. These are priceless and memorable moments that they'll surely be looking back on with a smile on their faces.
Key West is a really cool port of call on Disney Cruise Line's Bahamian itinerary. It's such a fun and laid back stop that my family visited last year. We had so much fun biking around town and we got a good dose of Key Lime Pie too.
What a great plan for a birthday rendezvous with your princesses at Key West! That won't be a private island stop, so you should be able to time your arrival to meet up with the ship's arrival at port. I'm sure your girls will be ecstatic to see you for the day. Disney's private island is
Castaway Cay and depending on their itinerary, that may either come before or after their stop at Key West. Unfortunately, you cannot meet up with them at Castaway Cay, since that port of call is reserved for guests cruising with Disney only.
While at Key West, there are several Port Adventures that are available and you can view them on this
link. You'll just need to choose Key West as the destination and the Disney Cruise Line site will give you a list of all excursions that are available for the day. There are a few choices depending on what your girls and you in-laws' interests are.
I hope this information helps, Liz. Please do let us know if you have any additional questions that we can try and answer. We're always happy to help.
Have a great day!
~ Dolly