Here's a challenge: write a story in 600 words or less, one that'll make me laugh or cry (or, even better, make me do both in quick succession). One with fully developed characters, a mature plot and a complete story arc. And then make it the kind of story I'll want to read six dozen times in as many nights, laughing or crying each and every time.
That's what makes writing a good children's picture book so challenging.
It's also what has helped me become a better novelist. No writer agonizes over every single word - not just the meaning and appropriateness of the word, but the sound of the word as well - as the writer of the successful picture book. No author is so attuned to the melody of a story, the tightness of the plot, the bare essentials-yet-completeness of the characters.
Now, to be honest, I don't give my novels the same level of scrutiny as I do my picture book manuscripts. It would be impossible to finish anything if I did. But the habits I learned in writing picture books have conditioned me to more readily recognize weaknesses in my longer works, to listen to the sounds of the language, to write tightly.
Writing is, to mangle a phrase, ten percent inspiration and ninety percent constipation. We all know how hard it is to produce good writing. For me, developing the skills and habits to write a good picture book has helped (ahem) loosen the process of writing novels.
And if you don't believe me that shorter is harder, then why is it so damn difficult to synopsize your novel in just a few pithy sentences? Well, PBs are pretty much like that.