Adam Rex is someone whose career is pretty diverse. He started out doing primarily fantasy art, and only broke into the children's book industry sometime around 2006. His work is very painterly, but his take on the more traditional side of things is very pretty to look at.
His book covers, in particular, are very well designed. He even takes the barcode placement into consideration.
He stated once in an interview that his history in fantasy illustration, such as Magic: The Gathering, hurt his entry into children's books.
It's really cool to see the differences in his technique when handling both fantasy art and children's illustration.
Seriously though, go check him out. Even his website is cool at fun