Ive been to Nassau a few times, I agree, you should go on shore and then take a water taxi over to Paradise Island to see the aquarium in the Atlantis Hotel. Maybe go in a few shops, walk on the beach at the Sheraton down the street from the Atlantis, then go back on the ship and enjoy the pool while in port.
If your daughters were older, I'd say also go into town and look around and get a feel of Nassau. Just a thought, but you'll have a good time either way.
I also thought the Wonder was the nicest ship we have been on, the shows were excellent, and the service was great. Food on Disney was just average for a cruise line though, but you'll enjoy it anyhow.
Private island for Disney was the best...