We did this Itinerary last August. Our sons were 14 and 15, and our nieces were 13 and 16.
On Cozumel we did the Tulum ruins, while our nieces and their family did the dune buggy and snorkel. I think they had an issue with the door of their dune buggy staying closed and some of the snorkel equipment, so definitely check it all carefully before you begin. Other than that, they had fun.
As for the ruins, our experience was much more like RedSox than EilonwyWanderer. I can't speak to the accuracy of the information provided because I am not an expert like Wanderer is, but it sounded plausible to me. I was worried the ruins wouldn't be "big enough." We considered taking the even longer ride to Chitzeniza, but we were plenty satisfied with the extent of the ruins. They ranged all over. Our Mexican guide told us a lot about the Mayan culture. He said he was a Mayan descendant and actually lived in a town about 4 hours away, which he goes back to on the weekends. He did try to sell us some carvings and other little things, but only briefly and they were at least as good as the other souvenirs offered by the other vendors there. At the ruins, the guided tour lasted about an hour, giving us plenty of time to explore the ruins (and the local iguana) on our own, and do some shopping on site. The tour guide gave out headsets, which were great so you could hear him without having to keep right on top of him. There was a small beach at the bottom of some stairs. Our provided lunches were in a cooler and nice and fresh. Plenty of other food to be purchased at the souvenir stores also, as well as local beer and tequila.
In Grand Cayman, we did the Dolphins and the Turtles. We only had like 45 minutes with the turtles, but it was still great! There were dozens of huge turtles swimming in these open-air pens. Then there was a section with smaller tanks holding turtles 1-1.5 feet in diameter that we could pick up and hold and pose for pictures. Then there was a breeding building that had five turtles about 3 inched across, which were born yesterday. (literally!) So we got to see all of that before it was time to walk across the street to our Dolphin experience.
We chose the "Extreme swim" package, instead of “encounter” or “swim.” It allowed us to do all three parts. The first part was the “encounter.” We stood (a group of about 16 people) waste deep in water in a large cement tank and the dolphin came up to us 1 by one, it kissed us, we kissed it, we held hands with it, it talked to us. Then we went into a larger area with a beach (still an enclosed body of water though). We did the “swim” portion, which involved us swimming out the far end of the pond (two at a time; this was good as it gave you a chance to watch your family members do it), and the dolphin came up behind us, he put his fins out to us, and we grabbed on and he swam us back to the starting line. Then we did the “Extreme Swim” part. So we swam (again two at a time) out to the far side of the pond again, this time we were holding a boogie board. He came up behind us and put his nose on our feet and pushed us back to the start. He was able to move MUCH faster this time with us holding the boogie board under ourselves. It was amazing.
On GC, our nieces did the Atlantis Submarine, which they loved. A real submarine (as opposed to a glass bottomed boat) that went down to a reef about 100 feet down. Based on the pictures they saw some great fish. I will say, that at that depth, the fish don't appear as colorful, because the water starts filtering out the colors (starting with red). Also, I can warn you if you take this tour, be sure to brush your teeth well that morning. As you go deeper, all white things glow, and if you aren't careful, your teeth won't! This was a shorter trip, so they had more time for shopping or to go back to the ship.
In Jamaca, we all did the Chukka Horseback Ride and Swim with Dunn's River Falls and Lunch (FA16). This was my favorite of the three days. The horseback riding was great and included about 15 minutes riding in the ocean. That was a great experience. We were not very experienced riding horses, but they match each horse to the rider, and we all managed fine. The 45 minute walk on land was great "learning experience" before the time in the ocean. The guides were able to group everyone in our family together, so we felt like we got to see each other on the trip.
Likewise Dunn's River falls was a great experience. You climb up the waterfall from inside it, but they have engineered it to make it passable, and they keep the slippery algae cleaned off. My 70-year old mother made it up the falls, with the help and attention of one of the guides, and my younger niece who is sometimes skittish about things did great too. Only my older son was a little bummed because he wanted to "billy-goat" up the falls like the guides were doing, but they didn't let him. There is also a nice set of stairs right next to the falls for people who want to go into the pools or see/take pictures of their families without climbing the falls. There is also a paved pathway from the top to the bottom for people who can't do stairs so they can still see the beginning and the ending views of their families climbing the falls. So to me, this is a good trip for groups who have people with various levels of mobility.