When my daughter was 7 she always wanted to watch scary movies. After seeing this one I decided to let her watch it, and she loved it. It was her favorite movie until she was 9.
Hellboy does smoke cigars, but it's not overly done to the point where your thinking it's an ad for kids to smoke. His main swear word is crap which isn't that high on my list of things not to say.
Overall this movie is great for kids. There is some action in it, but absolutely no nudity, and the violence is at a mild comic book level.
I think Hellboy makes a positive mole model as his story tells how he was made to do evil, yet chooses to be on the side of good. It's all about his choices and choosing the right ones. He also shows great friendship and love by willing to sacrifice himself to keep those he loves alive.
I also like the positive message that even though Hellboy looks like a demon he doesn't let what others think about how he looks degrade his self esteem.