My kids have both cruised most of their lives, on
DCL and other lines (
Royal Caribbean and Celebrity.) Both prefer DCL. We feel like DCL does the best overall job with babies, toddlers, and young children, although there were things we really liked about both other lines with kids. Honestly, we felt like almost 3 was the hardest age to cruise with kids with, because they could see all the things the 3 year olds could do in the kids club, but they weren't allowed full access. (We ended up getting lucky with our almost 3 year old fully potty independent DD, and being given full access to the Oceaneer's Club and Lab, but I understand they aren't doing that much if at all any more.) DD had loved the nursery when she was almost 2, but she hated it as an almost 3 year old year. She always loved the Oceaneer's Club and Lab during Open House hours, and then once she got access. DS was not potty trained right at 3, so we actually took a break from cruising, because we knew people who had cruised with a not potty trained 3 year old, who was not allowed in unattended in kids club. They did not have a good cruise.
Caribbean ports we have done with children 5 and under on various cruises, most of these at least 2 or 3 times:
Cozumel - we always do Chankanaab with young kids; the dolphin interaction is great, the sea lion show is fun, and they love the beach and tidal pools. We went to the mainland a lot before we had kids, but the ferry ride can be rough, so while they're young, we're sticking with Chankanaab. (DS requests it every time because he adores it.)
Roatan (Honduras) - beach break plus island tour that included a stop at the bird and monkey park. I would do this again. But occasionally the monkeys bite, including someone on our tour (an adult.) We loved Tabyana beach - it's fantastic shallow water snorkeling, and we've done it twice.
Belize City - Altun Ha ruins and wildlife spotting on the Old River. (We did Lamanai and the New River before we had kids.) If you take kids to ruins, you have to expect you won't hear the guide's entire spiel. They will wander off when the guide talks too long in one place, and you will have to follow them. Even so, we found it interesting and worthwhile. By age 8 my son wanted to climb the pyramid, so one of us had to go with him and the other had to stay with the little one.
Costa Maya - We split the party here. DH and I went to ruins; DD stayed in kids club; DS went with DM to a fairly nearby beach. Everyone had a great time. My DM didn't think she could manage both kids alone at the beach, and DD absolutely loved being in kid's club, so splitting up was a good solution for us.
Jamaican - left the then toddler on the ship with DM; DH and I went to Dunn's River Falls. I won't do Jamaica without staying on a tour, and I'm not eager to take the kids off the ship here. DH and I have been a couple of times, and enjoyed it very much, but didn't feel like it was little kid friendly.
Key West - First time DM took DS through town while DH and I kayaked. Second time we walked around town and did the aquarium. The aquarium is old and not in the best shape, but DS got to feed and pet a shark. He loved it so much he wants to do it again next time we go.
Grand Cayman - Once we did an island tour that included a glass bottom boat ride and the Turtle Farm. The glass bottom boat ride was awful. Too long standing around in a too hot line, then being crowded on a too hot boat. The turtle farm was fun, and we could see that there was more than the first tour had us do. The second time we just went to the turtle farm, and did the full adventure park that includes the big pool, the snorkel area, and the aviary. I would like to go to stingray city, but the kids haven't been ready to try it yet.
Nassau - Again, we split the party. We left DD on the ship in kid's club; DM went snorkeling; DH, DS (8 by then), and I did chocolate making. We all had a great time. I really thought about Atlantis, and will probably do it next time with go, but DD is our daredevil who adores water slides, and she was going to be too little to do all the slides she loves so much.
Castaway Cay - We got a cabana. We all loved it. We would probably have had an equally fabulous time without a cabana.
We've also done 2 weeks in the Mediterranean with a 5 year old, but I don't think those ports are as relevant to the current discussion. I will say, it was a great trip. A lot of our friends thought we were crazy to take a 5 year old on a Mediterranean cruise, but we all loved it. I did have to work hard to pick activities in port that would appeal to a child, but it was completely worth it.
4 or 5 is a great age for cruising. We've got one cruise planned for later this year, and actually have been contemplating a second. You will love it!