Ack! You should have studied Free Republic on the topic. Literally dozens of reviews have been posted here. This is of course offered on the theory that 20/20 hindsight is better than no sight at all.
BTW... I would have taken a 9 year old under the same circumstance. For good or ill.
Were there pre-release reviews posted here? By writers who saw it before last night? I missed them, unfortunately. I've read discussions of Tolkien (and Tolkien vs Rawlings ad nauseum) but nothing specific treating of the actual violence of the film version.
If I had it to do over again, I would take my 9 yr old again. He just closed his eyes if the scene was too brutal. We were looking forward to it too much to not take him.
Of course, now he wants to increase our nightly reading time so he knows the rest of the Trilogy's story line.