I'm working on an assignment for a children's literature class and I need to find out what are the world's top selling fiction/non-fiction
That's a difficult one, since no one routinely maintains a cumulative list--at least, as far as I know--particularly if you are including books published in languages other than English. Publisher's Weekly does publish a weekly list of children's best-sellers (U.S. market), which you can access through our article databases, General OneFile and Wilson Omnifile at http://www.omahalibrary.org/resource-center (home access with an OPL library card. Publisher's Weekly did publish a list of this kind in 2001; I've emailed it to you. You could start with that as your basis, and then do some research on top-selling books since then. The article also explains the difficulties of collecting this kind of data.
A great place to ask questions and discuss questions of children's literature is the Child Lit mailing list--see info at https://email.rutgers.edu/mailman/listinfo/child_lit . You can join for a brief period, and the archives are also very informative. I vaguely remember someone asking a similar question in the past.
I'll keep looking and let you know if I find a better source!