I plan to introduce the movies in the Machete Order IV, V, II, III, VI
The theory is that it makes for a more cohesive story overall and almost eliminates you-know-who from the story.
For the full details, see http://static.nomachetejuggling.com/machete_order.html
Jonathan Liu actually wrote up the Machete Order on the blog last month. And I think it's a universal win, assuming your kids can handle non-linear storytelling.
I think I was 6 when I saw Star Wars for the first time, and I loved it. As per usual, it totally depends on what your kids can handle.
...there goes the planet!
Battle Beyond the Stars! Jon boy Walton, Robert Vaughn and George Peppard - what's not to like?
The Last Starfighter
I'll second the call for Moon - I love the soundtrack
Attack The Block (alien invasion) or Primer (time travel head messer) - if you want 'future' classics!
Twelve Monkeys or Brazil
I'm guessing you've already done things like Blade Runner, Close Encounters, ET, Escape from New York
Starship Troopers or Robocop (but only the first ones)
Total Recall or Minority Report for some more PKD action
I completely forgot the Jules Verne and HG Wells movies..
First Men in the Moon (1964)
Mysterious Island (1961)
20,000 Leagues Under the Sea (1954)
Journey to the Center of the Earth (1959)
Master of the World (1961)