Oooh, look! Thanks for the shove, Pat... I just found the source - Writing for Children and Teenagers by Lee Wyndham, chapter 15.
The story opening is, according to Wyndham, the first page. Now, don't ask me if it's the first half-page of a printed chapter, or the first page of a single-spaced, 1.5 spaced, or double-spaced manuscript. From memory, I had thought it was in the first chapter, but often first chapters in junior fiction are shorter than later chapters.
I find the biggest problem with adult fiction is how many authors spend an awful lot of time describing the setting, doing a bit of a build-up. For me, and obviously a lot of kids, this doesn't work because if I'm not hooked on something more than the pretty scenery by the end of the first chapter you've probably lost me. The story works best, in my humble opinion (and reinforced by Wyndham's commentary), when it gets off to a running start.
This book, even though it's targeted at writer's of children's literature is a brilliant read for any wannabe writer.