yea, It was truly amazing :). If you think that cartoon characters were dancing on floor in front of judges and everyone with contestants, as they called it LIVE animation, I don't think such advanced holographic effects are possible yet in affordable costs.
According to me, it must be vxf (a kind of video editing). It must be like this that animation was recorded before the actual performance by technicians and they when kids were performing live syncing with that animation, a very difficult and great imagination truly. They mixed that animation with their live performance on screen. For this, judges too had seen the performance on screen like we saw on our television screen. :)