When I was in Nassau, I really enjoyed the Atlantis Casino and Resort. The kids will love the enormous walk-through aquariums (and the adults too!). That would be the best thing to do, IMHO. The particular name of the shore excursion I took to Atlantis Casino and Resort was called "Discover Atlantis". If you take the water taxi across the chanel to Paradise Island, just remember exactly where the water taxi docked and go back to that exact location. When I was there(on Paradise Island), a Bahamian man on the street was misleading people to a competing water taxi.
And if you heed MacMom's warning about water taxis, you could also take the regular 4-wheeled land taxi to Paradise Island. There is a large causway bridge that connects Paradise Island to Nassau. But with my particular shore excursion, they only used the water taxis.
Secondly, I took a "Tour of Nassau and Ardastra Gardens" shore excursion on my first visit to Nassau. It was pretty good. Basically, a driver drove us around Nassau pointing-out interesting facts and landmarks. The driver made a stop a The Queen's Staircase, Ft. Charlotte and ultimately Ardastra Gardens and Zoo. The kids will really like Ardastra, and all its animals. The marching flamingo show is the highlight at Ardastra.
I heard the beaches in Nassau are all really good. I have to take other people's words for it, since I have not been to the beaches in Nassau.
I would avoid The Straw Market if I were to go back to Nassau. The merchants in the market, and the street peddlers nearby were just too aggresive, IMHO.
So, take care and have a great time on your cruise