I actually re-read the series a short time ago. I bought them for my Goddaughter, and I read her a couple of chapters a night for a month or so. Having recently heard the crys from the left that they were Christian propaganda (and having read them as a kid without realizing this), I was amazed at how well-written they are. There is tons of Christian symbolism that goes right over your head (and obviously any 8 year-old's head) unless you are specifically looking for it.
I have been reading the series with my 6 year old daughter, and she picked up on it fairly quickly.