My favorite is Mandalay Bay, which has its own sandy beach and lazy river--and inside the resort has the Shark Reef, which is family friendly. Mandalay Bay is also connected by tram to the Luxor and Excalibur, which expands your options of things to do with the kids. The Mirage and Monte Carlo also have fun, kid-appropriate pool areas (at the Mirage, there's also an adults-only area). The Mirage has the Secret Garden & Dolphin Habitat, which displays some of Sigfried and Roy's animals as well as, of course, dolphins. (It's not a large exhibit, and it's worth noting that some visitors are bothered by the small size of the habitat.) The Monte Carlo's pool area includes a lazy river and wave pool, as well as a dedicated area for children.