There are about a million replies to your question, all posted before. Search marketing on the main forum page. No one method works for every writer or every book.
If you've written a children's book, it will be difficult to market -- too much competition from recognisable names. If you've helped a fifth grade class publish a book, ensure you have written permission from parents to include financial agreements; as account holder you'll be responsible for distributing any royalties, issuing tax documents (1099 in the US, I believe) and paying your own taxes.
Good question, but the answer is multi-tiered and also depends a lot on your book's genre. I published a children's picture book for an ex-friend a couple of years ago. In order to market it I needed to know his target audience, but he didn't write it for a particular audience/age group so it was really targeted at a bit of a mish-mash of ages. That made it hard to market.
In the end I used three methods. I created a great picture book website, published an introductory book and gave it away free (price matching as a freebie with kobo would have been great, but w never got that far) and also used listings on free book sites. 30 sold in the first few days and received great reviews. However, I also wanted to use content marketing which, over a period of time, would have proven itself (it has with my own books) because his stories were very, very good. He couldn't be bothered. So it went pear shaped and I made a new ex-friend!