Well, my little one is 7 years old but I won't let her watch Sweeny Todd.
Scooby Doo on Spooky Island, the Mummy, the Mummy II, Scorpion King and cartoon violence is as far as I'll let her watch because those are PG movies and don't show a lot of gore and stuff.
With her autism, she would think the stuff in Sweeny Todd is realistic since it it not over acting and 'pretending' like she can spot on the other movies. That sort of thing is not right for her. Sometimes she takes things very literally. It depends on what it is.
I had to show her the 'making of' parts on the 2nd DVD for Chronicles of Narnia to prove to her that Aslan and the other talking animals were not real. I guess the guys in the CGI department of the movie company should be proud of themselves.
She also watches every 'making of' on all her DVDs so she knows how they made the Mummy and the Sandman on Spiderman and Scooby Doo too.
Heck she is good at computers, maybe she can become a CGI programmer when she grows up. She sure knows all about the stuff since she watched that.
And she sings the Corpse Bride songs while she plays with her dolls.