I do hold the belief that J.K Rowling should be ranked right up there with William Shakespeare on the "Measurements of Brilliant Literature" scale. With her tale focusing on a boy whose SCAR is a symbol of wonder, Rowling manages to make her own writing style without making it seem...deranged.
She has indeed disproved the old stereotype of "British humor", as each novel--as far as I have read--has more than one hilarious moment in them. Some may seem slightly gross, but appear to be only so in movie format, as the author is polite enough not to go into detail.
The book series DOES, however, have a recurring theme of violence, but only when necessary, or when merely to the readers pleasure (because we all wanted Draco to get punched in the face by Hermione. Admit it.) For example, one book features a large snake-like monster (that I wouldn't happen to remember the specific term for at the moment), which is brought down by Harry when he shoves a sword into the roof of its mouth, but again, it never goes into bloody detail.
Another concern is that the Defense against the Dark Arts teachers are practically their universe's version of the Red shirts--that makes sense if you watch Star Trek--because they are usually only for the job once, but I do recall that only one died, the other one was fired, and I still haven't finished the third. (Perhaps he quit?)
Anyway, the "Harry Potter" series is a fun and unique ride to take with your children without regretting--so saying, if you believe they're ready for EVER SO SLIGHT violence.