"I write for what's left of the eight-year-old still rattling around inside my head"

This quote speaks to the idea that we never ever really lose our youth selves. Even as adults, we still bring the very same hopes, dreams, and imagination that we had as children. Chris Van Allsburg is recommending that he still has the very same innovative spirit he had as an eight-year-old, which this is the source of his writing. He is saying that he is still able to take advantage of the very same innovative energy he had as a kid, and that this is what drives his writing. He is also recommending that this provides inspiration for him, which he has the ability to draw on his childhood experiences to produce stories. This quote talks to the concept that our childhood selves are never ever truly gone, which we can still access the very same creative energy we had as children. It is a reminder that our youth selves are still alive and well, which we can utilize them to develop something gorgeous.