The Last Airbender
First, a distinction. Avatar: The Last Airbender, the Nickelodeon series, is one of the finest television series of the 21st century, a stirring and beautiful epic that successfully tackled stories far trickier than its audience demanded. However, The Last Airbender, M. Night Shyamalan's butchering of it, is one of the worst films ever made.
Your kids may feel a strong urge to watch it out of some misplaced reverence for the series, but don't let them convince you. The acting is atrocious, the story muddled, the effects third-rate and the tone so bizarrely grave that you may question whether The Last Airbender was intended as the most self-effacing comedy ever.