The difficulty comes in finding reviewers that book buyers (of all kinds) respond to, that are also affordable to the independent publisher's bottom line. Unfortunately, some reviewers, like Kirkus, charge huge amounts to review a self-published book ($425 or $575 for express). And, I haven't seen separate fees for picture books, which take about 1-2 hour to read and review. We produce between two and three QUALITY books each year, so those prices start to feel more like extortion than reasonable fees for a service. Publishers Weekly is another effective reviewer however that does not cost an arm and a leg. They don't promise a lengthy review, just a blurb, but they do feature the best submissions for a full review... so if the book really is as good as you think, no problem! The cost: $149. This is much more reasonable considering the service provided and the results obtained. The websites don't include a fee structure of any kind for traditional publishers...
My favorite reviewers are those who do it for the love of reading and the free book they receive, and who also have a large enough following that the review can translate into sales. These combinations are tough to find, but worth the effort.