The Mandalay Bay Hotel and Casino is at the upper end of what you might want to do for a family vacation. The wave pool and lazy river offer hours of fun and shark reef offers up close encounters with plenty of species of marine life. Fast food is scarce but a tram can take you to the Luxor or Excalibur in a matter of minutes.
Cheap food options include Burger Bar and Slice of Vegas. Michael Jackson ONE is a show that works for kids although the price for a family four can get steep very quickly.