Here's a article that will get you thinking. Many books considered masterpieces today, received brutal reviews when they first came out. The list in this article mainly has novels, but you will find a few children's books. One being a book that is now a classic example of the best in illustrated picture books...Where the Wild Things Are.
Publisher's Weekly, 1963
“The plan and technique of the illustrations are superb. … But they may well prove frightening, accompanied as they are by a pointless and confusing story.” — Publisher’s Weekly, 1963
One year later...
In 1964, the American Library Association awarded Mr. Sendak the Caldecott Medal, considered the Pulitzer Prize of children’s book illustration, for Where the Wild Things Are. "In simple, incantatory language, the book told the story of Max, a naughty boy who rages at his mother and is sent to his room without supper. A pocket Odysseus, Max promptly sets sail...There, Max leads the creatures in a frenzied rumpus before sailing home, anger spent, to find his supper waiting."
nearly 50 years later...
“Each word has been carefully chosen and the simplicity of the language is quite deceptive.”
#1 Where the Wild Things Are by Maurice Sendak (1963) "Arguably the single greatest picture book ever created." – Hotspur Closser — Top 100 Picture Books #1: Where the Wild Things Are
and 19 million copies, and counting, later...
"A signed first edition of "Where the Wild Things Are," the classic children's book by late author Maurice Sendak, has fetched an eyebrow-raising $25,000 on online book retailer abebooks.com, thus making it one of the most expensive children’s books sold in recent memory."
"Abe Books spokesman... Richard Davies told the Daily News that for "Where the Wild Things Are" in particular, it was rare for these books to go for so much because a lot of the 1960s editions were mass published. To date, 19 million copies of the Caldecott Medal-winning book have been sold."