You will need to bring biodegradable sunscreen here because that's all they allow, but just in case you don't have any they sell it at the front. There's really no good beaches here, even though they tell you there is. The kiddie lagoon and park is very nice. It's a little saltwater pond with a sandy area around it. There's playgrounds, hammocks, and lawnchairs all around. If you have a little one this is the only place you will be spending your time. There's really not much else for kids. This place is extremely spread out. Plan on walking long distances. We didn't see much because we didn't want to walk the entire day. You can see caves, snorkel, hike trails, swim with dolphins, and tube in the lagoon (they say it's a lazy river, but that's not true - it's just on a different side of the snorkel lagoon).
If you really like snorkeling this isn't the place for you. It's a bare, sand bottom lagoon. There are some large fish here though. The lagoon is meshed off from large predators, so the fish inside tolerate snorkelers in exchange for their safety. Snorkelers are eveywhere! Snorkeling off of a shore or a boat is much better.
We weren't that impressed with Xel-Ha, we felt that XCaret was much better and much more worth the money. They do rent towels, lockers, and snorkel equipment. It was nice, but after visiting XCaret we probably would not return to Xel-Ha. Xel-Ha does have some beautiful scenery and there are fresh water showers everywhere to rinse off.