Where can my students write online book reviews?

Perhaps at bookarmy.com? I do not believe they have an age limit. Though the children would not be up for any of the prizes, most likely.

Barnes & Noble has them, but also requires an account – which is free, and they have little avatars you can use and you can browse the books and create a “Wish List”.

I enjoy Good Reads. But they would have to have an account.

http://teacher.scholastic.com/writewit/bookrev/

You can try Edu 2.0. My Writing About Film teacher has us write movie reviews on it. It seems fairly simple.

I was going to suggest Good Reads, too.
Not only can they write reviews, they can also keep track of books they’ve read or want to read.

Amazon.com or shelfari are two good ones.

I second third Goodreads. See this question for more on what it is.