There were two areas of real concern for me as a parent with this movie. The langauge is still a bit off for 6 year olds ... "dumb, stupid, bozos" are the more common ones. Those aren't words I want to hear them say. Not very bad, but a little off. Generally phrases like, "I didn't you hear what I said, you idiot!", or, "... kept me up all night working on your stupid project."
The thing that got me the most was the scariness - and it was one scene. Near the very end when everyone on earth turns into a robot like worker, the earth goes dark, and looks polluted. I thought the scene was a little too intense. It didn't last long, maybe five minutes for that whole scenario, but enough that I thought it would bother my kids. The bowler hat guy himself, who people refer to - wasn't that bad. He was silly enough in parts that I think his scariness doesn't really count.
I always don't think most kids will understand the movie - the whole concept of time travel can be pretty complex.