I have two girls, who will be 5 and 8 when we travel the same itinerary on APril 9. This is our plan, take from it what you will, and feel free to ask any other questions you may have- tis will be our third
DCL trip and our second on the Fantasy.
Tortola: DH and I had done a rent-a-moped, do it yourself tour when we were here pre-children. It's a neat island with some cool stuff to see in terms of hiking and exploring, but nothing that really grabbed us as super appealing for our girls. Our 5 yo can swim, but wouldn't be a serious snorkeler, so a boat based snorkel trip wouldn't do a lot for her either. We have opted to sign on for the "trip to the baths"- you take a ferry from Tortola over to virgin gorda,then get to explore the baths- a beach area where a collection of HUGE boulders form a sort of cave system you can explore and a nice beach area with swimming and some decent snorkeling around the rocks that tumbled down into the ocean. We did this on a pre-kids cruise as well and really enjoyed it. Our girls are pretty sure footed and are enthusiastic about the clambering on rocks and splashing through the little pools of water on the way to the beach. If you and yours are reasonably agile, they might like it too. (working with your "adventure" and "beach") Worth doing some searches to find out more. The DCL excursion seems a little pricey for what you get- I'm researching an option through Speedy's ferry, an outside vendor, which is about half the cost. Not sure what your comfort level is for that.
St. Thomas: here we plan to visit Coral world and Coki beach. We have done this before, with the girls, and enjoyed it so much we will do it again. There are a few different DCL excursions that offer a vist- you can look to see if any combinations of what they offer appeal to you. For this one we are planning to just take a cab so we have a little more time to spend both at coral world, so we can see all the demos etc., and time to actually enjoy the beach. While neither coral world or the beach are particularly large, some of the excursions seem to only allow a couple hours to do both- we don't need to hurry back to port for shopping or a second excursion so we's prefer to stretch things out here. Coral world also has different animal encounters and snuba and stuff as an add-on experience if you're looking for something like that. Also inexpensive upgrades like food to feed the lorikeets, etc. With regular admission there are touch tanks and demonstrations etc. so if they like animals this might tick that box. there is a cafeteria here if you want to buy lunch. After exploring, it's a quick walk to coki beach next door where you can rent a beach chair, umbrella if you need it. there are roaming vendors offering food and drink from small restaurant shacks Also option to rent scuba gear tough we just brought our own snorkels. decent things to see in the water- people regularly feed the fish here so they're plentiful and an"easy get" for the kids if they want to try it out. When done, it's not hard to find a cab back to the port either right at the beach or by walking back to coral world. I don't remember whether this was one of the places where we had set a return time with our initial driver or whether there was just a couple cabs waiting around (it's a fairly common destination for cruisers and other tourists).
Castaway: we've never bothered actually booking anything here. I like to run the castaway 5k so I sign up for it at guest services, (it's been free though I worry that will change). The rest of the family gets off the ship at their own pace and sets up by the beach not too far from the waterslides. They come to cheer me at the finish and then we just play it by ear. We've rented a couple bikes to ride around and explore, done some snorkeling, the waterslides, and just relaxed and hung out. there is a kids club on CC called scuttles cove so if the kids like group activities you could have them check that out and explore the adult side of the island- this is on my agenda, we haven't yet tried it. But by the end oft he week you may not have a lot of "go, go, go" left. Maybe someone else would have more to say.
So that's my plans/ experience. Take it for what it's worth.