Donna and Robocop have given you excellent advice on getting off the ship at every port on your itinerary. I have visited each of these ports and you MUST get off the ship. This itinerary you have is an interesting one. There is a lot to experience on this cruise: different climates, different cultures and different languages.
First of all in the winter there is about a diference of 10F between St. Thomas at 70F and the lower Caribbean 80F temperature. Aruba and Curacao are arid, desert climates and the wind blows continuously. In Aruba, the DiVi DiVi trees are permanently leaning over due to the warm winds. Both Aruba and Curasao are a part of Holland and the architecture of the buildings reflects this heritage. Dutch is the official language but, English is spoken widely. Curacao has a unique feature-the pontoon pedestrian bridge over the ship channel. The pontoon bridge swings open to let the ships pass to a lagoon where tug boats swing the ship around, then assists the ship to the dock. Once you disembark the ship you walk over the pontoon bridge to get to town. Have your camera ready as your ship comes into this channel because the buildings are a sight not to miss and demand a few frames of your film. Food is imported to both Aruba and Curacao by boats from Venezuela which is about 35 miles away. In Curacao the boats tie up right downtown against the seawall and set up colorful fruit and vegetable tables under brightly colored overhead awnings. Shopping is good for Dutch products so save some time at the end of the day to do your shopping. With 5 people you might want to just hire a minivan cab for a personal tour rather than pay for 5 excursions at Aruba and Curacao. The cabbies are good guides. But settle on price and make sure he speaks English enough to understand him. Don't be timid here!
St Lucia is it's own country and you will probably be docking in Castries, the capital. St. Lucia is a tropical country and in addition to tourism, bananas are a major income crop. Their bananas are shipped to Europe. There is not much to see right down town near the docks. Here you might want to also hire a minivan cab for a short island tour of banana plantations or take a tour boat south to the extinct volcanos, the Pitons. This is a beautiful site and there is swimming there. Outside Castries is Bagshaws Studios. Sidney Bagshaw started a silk screening business of his artistic drawings of the flowers and birds of the Caribbean. This is all done on Irish linen and the designs are exquite. Bagshaw has a retail outlet at the ship dock at Point Serafin if you don't have time to visit the studio and watch the silk screening made first hand. This is not a comercial for Bagshaw, but I love the wall hangings and placemats from his studios and I love good design. There are many things to do here, but they are spaced out quit a bit. You will have to make a decision on what you want to do.
St. Thomas is the shopping mecca for the islands and it belongs to the USA. There are various tours of the island, but hiring a minibus cab is in order here. This is a part of the USA, but they drive on the LEFT side of the roads here. Don't even think of renting a car. Leave the driving to THEM. But, watch out for traffic coming from the other direction when you cross the streets!
There are beaches at all of the islands you will be visiting on your cruise so you should have no trouble getting plenty of sun and surf. I have just tries to enlighten you to some of the cultural adventures you can add to you family trip. Partaking of some of the sights I have mentioned will give you a bit more depth to your vacation. Remember, three of these islands are foreign countries and have interesting customs and lifestyles to share with you. Absorb all you can and take a lot of pictures for those memory trips in the years to come. Bon Voyage!