You already know about tight writing, words that sing and dance, visual pacing, picturability. I'd like to add depth to the mix. If a parent is going to spend a whopping $17 on a book, you want to make sure it has that special something that a kid will want to read over and over and over again. It needs to work on many levels -- entertainment, educational, even nostalgia. That said, I have read a few PBs that were originally magazine stories.
Good luck, Franzilla.
Vijaya