I love children's books & read them as lot. I think that if authors had to be as careful with adult books as they are with children's then the standard of literature would be higher: by that I mean, keeping the plot tight, making every word count, being clear.
I re-read a lot of my childhood favourites, and others that I've acquired along the way. I am a great reader of myths & legends, many of which get written up as "children's literature". My very first "can't put this down" experience was age 6, the children's version of The Odyssey, and I have to say that I've been hooked on "band of brothers" stories since.
I began to read adult books as a child - my parents & grandparents thought that if you could actually read it, it couldn't be unsuitable (!) and by my teens I had read a lot of classics, including Dracula & Frankenstein, and a lot of romance authors from Victorian times up to the 60s - I am still mildly astonished at how protracted Victorian death-bed scenes were (still a fan of Charlotte M. Yonge)
The Faraway Mountain was by Lynne Reid Banks: Radio 4 recently re-did the L shaped Room as it has ben 50 years since publication (I read that at about 12!)
So for me, since about 8, the boundaries have only ever existed in other people's minds!
And for me, sorry as I am to have to cede top position to a non-English language author, Astrid Lindgren is wonderful!