One website that's good to read is goodtoread.org, a parent's guide to children's books. Here is part of what the site's creator says about his approach:
Why another site about children's books? One answer to this is easy: you can't have too much of a good thing. Why have more than one sweet shop on the High Street? They all sell much the same things at much the same price, but perhaps you find the people in this one easier, or the display in that one more appealing. There are many websites that give information about children's books, and there are many people with different tastes in websites.
On another level, this website offers a slightly different approach from the majority: it doesn't assume that a well-structured book with a good spread of vocabulary and a certain depth of subject matter is unquestionably a book any child should read. At goodtoread.org we believe that children and young people can benefit from some guidance as to their reading. This may mean recommending a book especially or discouraging a certain book or author on account of the poverty of style or unsuitability of the subject matter. Especially when a series is popular or often recommended, it might mean going over the subjects presented in the book with your child to make sure the youngster has benefitted from it and not been harmed.
To read the whole page (and it's a page worth reading), click here.
All this sounds very good but the test is what happens in practice. A useful test case might be what goodtoread.org makes of Philip Pullman's His Dark Materials trilogy so here's the link.
That seems to me to be a very fair-minded, balanced and honest appraisal from a thoughtful Catholic so I'd heartily recommend the rest of the website.