"A truly great book should be read in youth, again in maturity and once more in old age, as a fine building should be seen by morning light, at noon and by moonlight." Robertson Davies
If Davies' analysis of what makes a truly great book is correct, then the subject matter must be safe enough for children and be conveyed with simple plot and plain language. However, the characters and the psychological underpinnings should be sophisticated enough to hold an adult reader's attention. Are there books that fit this description? I would suggest that if adults would consent to reread books from their childhood that they think of as children's books, they might find it is true.
Some American books that I think meet this criteria are the Little House on the Prairie series by Laura Ingalls, Charlotte's Web by E.B. White, Little Women by Louisa May Alcott, The Time Quartet by Madeleine L'Engle and The Adventures of Tom Sawyer (and The Adventures of Huckleberry Finn) by Mark Twain.