It is a challenge, but engaging with the writers' community on social media allows you to gradually build your knowledge on who blogs about (and reviews) the kind of books you write. As you build mutually supportive relationships with people over time, you can find out about people and sites who may potentially review your book, and you can ask if they'd like to receive a complimentary copy for review. This takes time and effort. It is my opinion that, within the kidlit community, glowing PAID reviews are not really respected, nor worth it.