The link takes you to a login page requiring a username and password. Check with your local school district or state education department. Several states and many school districts have a partnership with MyOn. For example, where I live, I have access to over 5,000 educational books through MyOn. Many of them are perfect for my 9 year old. She has her own Kindle reading device now and she can access these materials on her own.
Your district or state may not have a partnership, but it's well worth it to investigate and ask!