I agree with all the nighttime shows (Wishes/SpectroMagic at the Magic Kingdom, Fantasmic! at MGM, and IlluminNations at Epcot). All of the shows are amazing. And also, if they offer it when you're there, try the extra magic hours at night- I was there over spring break (one of the most crowded times in WDW!), and my dad and sister stayed at MGM for this. They said that the crowds really were smaller, so it's a great opportunity to do some of the big rides with much shorter waits. Enjoy your trip, and I hope your niece has a great time!