I think the definition of a good writer for children is very different to a good writer of adult fiction, and the issue with so many adults reading the Harry Potter books blurs the line there. I also think there is a huge difference between being a good writer and a good storyteller - most children's book authors to my mind succeed by being good storytellers, not necessarily good writers.

I always use Tolkein as an example of this. I think he's an amazing storyteller, the world building alone is phenomenal, not to mention the mythology and linguistics - but a good writer? Not for me.