Keep an eye on her blog, where she's posting outtakes from the essay -- full interviews with lots of YA and YA-crossover authors.
From her interview with Mark Haddon:
Nevertheless… for a long time it was possible for an established author to write a children’s book and everyone think it rather charming. But an established children’s writer trying their hand at a novel was considered gross presumption. As my previous agent once said to me, ‘Stick to what you’re good at’. Underlying all of this was the idea that children’s books are easier to write. They aren’t. As a number of adult novelists have proved (no names, no pack drill) they aren’t.