I’m a huge fan of Luke Chueh, Joe Ledbetter and other such artists… but their art is more appropriate for an older person’s room rather than a childs (though one might argue that for Ledbetter).
Fall Down Creative (found via DesignMom) has some cool art that while not quite as edgy as Chueh provides an acceptable alternative in my mind.
If you’re looking for a fun print to fill an empty wall give Fall Down Tree a look… but give me a few minutes to grab a copy of Three Mice That Are Blind before you do.