Amazon.com and Goodreads are good websites to find online reviews of children's books. These websites have a large readership and rely on user-based content to advise visitors on what they should read or purchase in the future.
Amazon.com is a consumer-based collection of reviews, whereas Goodreads is a social media website devised for readers. As a result, a person going to Amazon should be aware that some of the reviews pertain to shipping or formatting problems. Before making a decision on content based on the reviews, read through the outliers to see if the complaints are pertinent to the book itself.
Goodreads provides a service that is completely user-based. If you are looking to connect with other people who share your taste in books, this is a good resource. You can add people as friends who read the same books as you, and their reviews show up in the news feed on your home page. As a member, you can even join groups dedicated to children's books and participate on forums where discussions of the merits of certain books take place. This is a good way to get a much more personal review of a book that you are considering reading to your child. Both websites are free to access, though Goodreads does require that you create an account to get the full benefits.