One of the great misconceptions regarding books for children - and by this I refer primarily to fiction - is that they aren't as hard to write as those for adults. The reasons for this common belief are twofold:
1. It would seem that children have smaller brains, and therefore tend to use smaller words. This is taken to mean (by some) that the writer for children doesn't have to worry about all those great big words that "real writers" use.
2. So much high-visibility children's literature (very loose use of the term) is crap, thereby lowering the way the rest of the genre is viewed. These books (you can find them advertised in junk-mail and at supermarket checkouts, mostly) are patronising, cynical and of no real benefit to the child, and their existence is both testament to and a contributing cause of point 1, outlined above.
Having said that, many writers of books for kids do in fact feel that working within that a genre is easier than writing for adults. This is not because they are lazy and looking for a quick buck - it is because they possess the rare ability to write in a voice that children relate to and can embrace.
There is a third misconception, being that writers for children are only doing so for a short while, until they feel ready to tackle a "real" book, ie a work of adult fiction. To the many people who think this, I would refer them to three writers, each of whom is best known for their children's books. Between them they found success in every measurable way. If you can forgive the hollow cliche, Roald Dahl captivated children of all ages (and continues to do so). C.S. Lewis created a world which, along with the world created by his close friend Tolkien set so many children on the path to fantasy. J. K. Rowling's wealth and universal celebrity is now the stuff of publishing legend.
Are children's writers working within that genre as a stepping-stone to mainstream publishing? Some are, of course, but most are not. This latter group understand the importance of adults who can drop at will into the mind of a child and stir up something from their own childhood experience that will resonate with the young reader.