Beeghly Library maintains online subscriptions to a number of professional book review magazines. Some focus exclusively on children's literature, while others mix children's and adult literature.
There are also plenty of high-quality free review sites on the web. Here is a small sample of them.
The following two sites allow users to review, keep track of, and categorize all of the books that they have read. You can read what other people thought of what they read and keep track of your own reading.