We went to Parque Nizuc with three small children (3, 6, and 9) and it was absolutely excellent. It was expensive, but I felt that we got more for the money than we ever did at Disney World.
The Dolphin Interactive Program was fabulous, even with a six-year-old. They got to examine and pet and do tricks with a real live dolphin, who was very gentle and well-trained. The trainer was very knowledgeable, and spoke very good English. (They alternate English and Spanish parties.)
One thing that I wish we had known is that friends and family can stand on the dock above the Dolphin Interactive Program. My littlest one and her mom went off to play, and missed a good time.
There was plenty for the little ones to do, and they had a blast. Many of the attractions require the kids to be at least 42" tall, so older children will have a wider range of activities. There were plenty of life guards, and they all seemed serious and well-trained. Everything was clean and well-maintained.
Photos of the dolphin kissing you on the cheek run about $12 EACH. Be prepared for a little sticker shock (as with most things in Cancun).
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.