We went to Xcaret last week-9/20. We took our five and seven year old daughters. They enjoyed it a lot! The underground river is neat, but my husband had to drag our five year old through it. It's dark and a long way. Our seven year old swam the whole way by herself. It's really long even for adults, but the cool water is refreshing!
It's a neat park. It's part zoo, aquarium, beach. The lunch was ok. Choice of hot dog, hamburger, etc with chips and a drink. The free ice cream was not really a choice--you had to take the one they offered.
We snorkeled--bring your own mask, vest, and snorkel if you can. You probably won't be able to see even half of the park in the time you're there. You'll need to prioritize. We did the river first, then ate and snorkeled. We went through the aquarium (air conditioning), and wandered through other animal exhibits. That's really all we had time for.
We took a tender boat from the boat right to Playa del Carmen. I heard that now they are docking, so that's probably changed too.