Hi again, Erin!
Your cruise with port stops in Key West and Castaway Cay will be such a nice escape for your family!
I have not rented bikes in Key West before, but I have heard great things about Eaton Bikes on the island if you want to look that up as one option. Alternately, Bike Rentals Key West is another option. Both places have advertised the possibility of delivering bikes to you at the pier if you pre-book, so if they are still offering that, you might consider it. Check out their sites for more information, though, as they are not affiliated with Disney Cruise Line. In looking at their sites today, I do see quite a wide variety of bike options, including child seats and trailers.
You might not know that there are
bikes available on Castaway Cay as well. Spending an hour or so biking around Castaway Cay's bike trail is a really nice way to spend part of your day on Disney's private island, Erin! There are bikes with child seats attached and the weight limit for those seats is 40 pounds. There are also numerous small children's bikes with training wheels if your three-year old wants to ride on his or her own.
I hope that helps, Erin! Just a few more weeks until you'll be enjoying your cruise! Happy planning!