As an author, I found the links on Author Resources of The Kindle Book Review very helpful. Especially as I prepared a KDP Amazon promotion with The Snake Who Wanted To Be A Horse.
Then I discovered the Get Reviewed page which lists book reviewers. There wasn't a reviewer who specialized in Children's Picture Books, so I applied. And was accepted! You can see me on that Get Reviewed page. If you want to have your book reviewed, then follow the precise instructions and make sure you only email it to one reviewer at a time. A reviewer for Kindle Book Review posts their review on Amazon, and because there's so many quality and helpful reviews from that one account, these reviews have more weight than an everyday reviewer. You need to understand how reviewing works with Amazon in order to understand what I mean.
How does Amazon reviewing work? Anyone who has an Amazon account can review the books they buy. After a certain amount of reviews that are voted "helpful" (I'll explain that in a minute) then anyon can reach a point where they can review any book, even if it wasn't bought through Amazon.
How are reviews voted helpful? At the bottom of each review there's a question: Did you find this review helpful? Yes or No. When you click Yes, then that reviewer gains "status." By status, I mean that his/her reviews are given more weight by Amazon, because other people have found them helpful. The more helpful votes, the more weight the reviewer is given. In fact, some reviewers become Top Amazon Reviewers. These are the reviewers that we, as authors, want to review our books!
Here's a guide that goes more in depth for about contacting those top reviewers. Keep in mind that the very top reviewers receive a great deal of review requests, so you might have better luck with reviewers lower down on the list. Also, my advice is to choose a reviewer who reviews books in your genre. You'll have better luck getting a review.