I doubt you'll have much luck getting your hands on these. I know if I were the parent of such a child, I would never hand over his/her journal entries to a writer. Never!
Maybe you could find adults who used to be those children willing to share their childhood journals, but I frankly wouldn't think you'd have much luck there either.
I'm a writer, too, and my time period is 1000 years ago, so I sympathize with not having as much source material as you would like. But I don't know what to tell you. If it's not posted on the web or volunteered to you by people who know you, I doubt you'll find much. It just has the potential to be too explotative. You may just have to speak with psychiatrists in generalizations and invent the rest.
Maybe post with SENG? Maybe you'll catch someone's eye there?
