Kick-Ass is one of those movies you'll probably either love or hate. If you take something like this film out of the realm of satire and try to look at it from a real world perspective, you will probably find it not only offensive, but a horrible influence on younger minds. But if you do that, you're missing the point entirely and I'd suggest that you skip it. Nobody is forcing you to see it. And you can object in the comfort of your own home by changing the channel when you see a movie trailer. If you're old enough to understand what Kick-Ass is about, but too old to see it the rampant violence as anything but rampant violence... again, I recommend you skip this film.
Obviously this isn't a movie for kids under a certain age. I suspect that age may be the middle teenage years, but your mileage may vary based on the kids involved. If you don't think your kids can handle separating fact from fiction and reality from satire, then this isn't a movie for them. It's that simple. We don't need any kids running around thinking they can fight crime in costumes. The results would vary from minor injuries to death, and it's kind of tough to recover from that.
That said, what is this movie about? Kick-Ass is a movie based on a comic book series written by Mark Millar and illustrated by John Romita, Jr. that was published by Marvel Comics under their Icon imprint back in 2008. The story focuses on teenager Dave Lizewski (Aaron Johnson) who decides he'll take a crack at becoming a real life superhero called "Kick-Ass." Lizewski manages to get his butt seriously kicked his first time out, but that doesn't deter him from doing it again. And when he gets caught on amateur video, he's an overnight sensation.
Lizewski is enjoying his new fame so much he doesn't notice he's getting pulled into a bigger war between a crime kingpin, Frank D'Amico (Mark Strong, Sherlock Holmes and the upcoming Robin Hood) and an ex-cop turned vigilante, Damon Macready (Nicolas Cage, National Treasure and the upcoming The Sorcerer's Apprentice). Macready has donned the persona "Big Daddy" and dressed himself up as a wanna-be Batman while taking out parts of D'Amico's operation a bit at a time. Along the way, Macready trained his 11-year-old daughter Mindy (Chloe Moretz, (500) Days of Summer) to be a lethal killing machine herself, donning the persona of "Hit-Girl."