Here's some older books for free for a variety of Korean age and grade levels: http://www.koreanlab.co.kr/main/index.asp?t_id=1 You can also visit some Korean children's websites and just channel your inner child. When you're learning a new language and culture, you're absorbing them and molding a new part of yourself. And that part of yourself starts off as a child^^ And it spends a lot of time verbally as a child. I have some more links for these types of sites, but they're on my other computer. Just do some searching. For example, Disney Jr.

I know LingQ.com has some reading material in Korean (among other languages), including a small amount of children's stuff. Reading them on line is free. To take advantage of audio and other advanced features, you'll need a paying membership.