While this film is definitely not for kiddos, it is, by today's ultra-violent standards, not the shockathon it seemed to audiences back when it first came out in 1967.
The violence is quite real, though, and we feel deeply when some of these characters—flawed people we've grown to like and care about during the story—are killed. The exception is the film's last scene, though. Even today, nearly fifty years after people first saw this film, this scene is really and truly jolting.
So yeah, don't screen this one at your 6-year-old cousin's birthday party.