I haven't done a cruise with 1 day stop in Port Canaveral.
However, never having visited and having a 5 year old, I really think you'd want to focus on the Magic Kingdom. That really is the center of the Disney experience for most kids (and a lot of adults, too).
It's my understanding that the last buses leave late enough that you will be able to enjoy the evening entertainment - including Wishes, the fireworks display, and the Main Street Electrical parade. (Assuming the parade is offered on the day the ship will be stopping at Port Canaveral)
Since you've never been, I wouldn't waste time park hopping. It can take 30 - 60 minutes to get from one park to another - and that includes STRICTLY the time from the moment you get to the bus stop until the moment you get off the bus at the next park. That doesn't include the time it's going to take to get from wherever you are in the parks to the buses out front. Including all that, I think you're looking at a good 60 - 90 minutes depending on crowds, bus schedules and traffic. And again - that's from one park to another. You're looking at that chunk of time if you park hop multiple times.
My own, personal, Magic Kingdom top 10 list:
1.) Splash Mountain
2.) Haunted Mansion
3.) Big Thunder Mountain
4.) Pirates of the Caribbean
5.) Wishes
6.) Peter Pan (ride in Fantasyland)
7.) Main Street Electrical Parade
8.) It's A Small World
9.) Space Mountain
10.) Jungle Cruise
Once you book your cruise, when you book Port Adventures, they'll have more information about bus schedules for you, so you can see what time you'll need to get back to the ship.