@Zavellart I don't have experience in selling books, but I do in selling other products. The essence of marketing is finding the group of people that want your product, that your product is made for, and reaching them to offer them your product in a language that addresses what your product will do for them. So in this case the target audience is children, yes, but children aren't buying it - parents are! So where are parents hanging out online? Maybe parenting Facebook groups? Mommy blogs? I bet Pinterest is a big one! Next think of how to you get in front of them using in these places, what's the best way. A blog feature, and ad, maybe pinning on group boards? Finally, think about what they're getting from your product, from their perspective, what problem is it solving for them. Is it an entertaining bedtime read that will capture their child's imagination? Is it a great conversation starter on bullying or depression?
Here's a bad example and a good example.
Bad: Make a post in an artist Facebook group with an Amazon link and the caption "My first book is out, go buy it!"
Good: Make a post in a parenting Facebook group with an Amazon link and a caption: "It can be really tricky to start conversations with young children about such a serious and sensitive topic as depression. But sadly, depression is a part of your child's world and those hard conversations are so necessary for your child to understand and learn to navigate it in a productive way. Using my own experience of talking with this with my children, I decided to write a children's book to help parents navigate this topic. The book tackles the subject in a sensitive yet charming way, introducing it to your child at their level to help them understand and naturally start a conversation about depression. The book is available for a limited time at a reduced introductory price of 30% off!"
The first example is the wrong audience and a wrong message. The second example is the right audience, and a message that is all about THEM and what the book can do for them, not about you. A limited time offer may also push people who are considering it or on the fence, encouraging them to make a purchase now instead of putting it off.
I hope this helped even a bit!