Direct answer
Xcaret Park for a full day. Cenote Cristalino for a half-day. Akumal Beach for swimming with turtles. Mayakoba bike tour for a quieter morning. 3rd Mall on rainy days. Skip the dolphin programs — there are better experiences that don't have animal-welfare concerns.
For high-energy days: Xcaret Park (cultural shows + cenote swims + animal habitats; budget a full day). For half-days: Cenote Cristalino (open-air, easy for kids) or Akumal Beach (snorkel with sea turtles). For lower-key: Mayakoba bike tour or beach time at Mamita's. Most kid-friendly restaurants on 5th have play areas.
Local context
Playa is more family-friendly than its adults-only-resort marketing suggests. The beaches are flat and calm (especially north of Coco Beach), the cenotes are kid-magnets, the Mayan ruins are climbable, and the food culture lets parents eat well without hunting for a kid-specific menu. The big trap is over-indexing on Xcaret-group parks — they're well-run but cost a fortune and you'll have a better trip mixing one theme-park day with cenotes, Akumal turtles, and a Cozumel ferry day. Sargassum (seaweed) is a real consideration April–August on certain beaches; check live cams the morning of beach plans.