As has been noted many times, this book is the most mature and darkest in the series up to that point. However I felt like the movie eliminated all the aspects that made the book marginally acceptable for younger children. The heart and soul, the morality of the novel was simply avoided in the film ... the scene in which Malfoy's mother begs Snape to protect her son was stripped of the emotion that was written into the novel. Dumbledore's explanation of how the power of love and loyalty overcome evil in the world was simply deleted from the film. Malfoy's struggle with good/evil was explored only very shallowly in the film. The one thing the movie stayed true to was Rowling's apparent obsession with alcohol in the 6th book (although the movie deleted the apparently alcoholic Professor, thankfully). I felt like this movie was simply an annoyance to the film makers -- it seemed to be made only because it was necessary to set up the next installment of the series.
By normal teen movie standards, this movie is tame but parents should be aware that there is mild violence (actually fewer "battle" scenes than in the book), and making out and kissing features prominently throughout the movie, as does alcohol. Hagrid and Prof. Slughorn get drunk and Slughorn serves Ron and Harry alcohol.
There is also one particularly scary scene involving zombie-like creatures which is disturbing for younger children. The PG rating really surprised me. I think kids who have already read the book without being scared will be able to handle the movie, but honestly this movie doesn't do justice to the novel. If you really want to understand the mythology of Harry Potter and Lord Voldemort or to have a meaningful discussion with your children about the struggle of good and evil, choices and consequences, and free will, you'd be much better off reading the Half Blood Prince aloud as a family rather than seeing this movie. I feel for the film makers having to adapt a 700 page novel, but this movie is simply too shallow to leave any kind of lasting impression. I left the theater feeling sad about the lost opportunity.
What other families should know
Too much drinking/drugs/smoking