Aims, I'm going to defer to Neil Gaiman on this one:
Hello Neil:
I am planning in taking my 6 year old boy for his birthday party to see your new movie coming out soon "Coraline' I was wondering in your opinion if this film would be too scary for 6 years old.
Thank you for your response.
Graciela Jenkins
...and the only real reply I can give, is that it's a bit like saying "I'm planning to cook a mushroom omelette tomorrow, and do you feel this food would be welcomed by a six year old?"
Answer: I don't know. I don't know your six year old. They tend to like different things and respond differently. Does your six year old like mushrooms or omelettes?
And the answer to is Coraline right for six year olds is, I don't know. What sort of thing does your six year old like?
I think a good rule of thumb would be, that if your child can cope with The Nightmare Before Christmas and the original Wizard of Oz then they should be able to cope with Coraline just fine.
As a general rule, Coraline the book is much creepier for adults than it is for kids, who tend to read it as an adventure. I suspect that this will be true of the film as well.