I would absolutely love to add more King children's books to my library! First, I'm a completist as well and try to get my hands on anything King. Also, I can't wait to introduce my little one to King. I started reading King's works when I was 9 and had no "children's book" introduction. Dove in with Insomnia and quickly found that I liked drowning in his books! I have two copies of Tom Gordon - one specifically saved for my soon-to-arrive child. I have a pretty good recollection of which of King's works are a bit "softer" and could help ease a child into reading, so I plan to read aloud a few of those early on before letting the kid go wild on their own.
I'd especially like a Dark Tower children's story with Oy... since that inspired our nursery theme.