There are basically two different kinds of movie trailers. The ones for all audiences are known as "green band" trailers. You'll even see a green background when the trailer begins. Then there's the "red band" ones. These are ones that tend to have more 'adult' content. Let me just say that this trailer is not for everyone. If you are easily offended, you might want to just keep moving along. Do not click play. Empire Online had this trailer earlier and now IGN has this embeddable copy.
(Not sure if it's only viewable in the U.S. Let me know if that's the case. Otherwise click on the Empire link above.)
Hit-Girl is played by Chloe Moretz. Kick-Ass, as you may know, is a movie based on the comic by Mark Millar and John Romita Jr. It's due in theaters on April 16, 2010.