Have only been to 2 of those locations, St. Croix and St. Johns, Antigua. Between the 2, the better diving, imo, is in St. Croix.
In St. Johns, the cruise ships dock right in town with lots of shopping and a casino. You would have to take a taxi to get to a dive op. If you do wind up diving there, I dove with Indigo Divers.
Scuba Diving in Antigua They leave from the boardwalk in Jolly Harbor and everyday I dove with them they had people from the cruise ships. They have a nice clean boat and take a maximum of 8 divers. If not diving and you want to see the islands hot spots, a taxi driver (you can find names/numbers on TripAdvisor) is a good option and probably cheaper than a cruise ship excursion.
In St. Croix, the cruise ship will dock at the Frederiksted Pier. The nice thing about this stop is that if you dive, you are on the west side of the island which is the calm side.
At the end of the pier is N2theBlue's dive op.
N2theBlue Scuba Diving St. Croix US Virgin Islands
I'm partial to them as I have always stayed on the west side. They are a great op. They only take 6 divers out in their boats. Close to the pier are wrecks and reefs and you can probably request what you would like to see. As
@KathyV indicated, the pier is a fantastic dive but probably out because of the cruise ship. Since this would be a convenient and great place to dive that leaves finding something for your family to do. Vendors will have tables and tents set up with items for sale and there is a small fort they could look at, but there is also, somewhere behind N2theBlue's dive shop, a place (the name is eluding me right now) that does ATV tours. A short walk gets you to a beach area that probably would have some good snorkeling. Many people take a taxi to the other side of the island to Christiansted where there are actual shops in town.
Wish I could compare the other spots. Hope you can dive St. Croix. Don't think you'd regret it. Have fun and let us know what you wound up doing when you get back.