“The American child is a highly intelligent human being — characteristically sensitive, humorous, open-minded, eager to learn, and has a strong sense of excitement, energy, and healthy curiosity about the world in which he lives. Lucky indeed is the grown-up who manages to carry these same characteristics into adult life. It usually makes for a happy and successful individual.”

"George and Gide"
Two Cheers for Democracy (1951)

Source: Living Systems: Basic Concepts (1969), p. 53; About conceptual or abstracted spaces

Alick Bartholomew: The Schauberger Keys

Introduction
The Acts of King Arthur and His Noble Knights (1976)

Pt. 1, Ch. 5
Christine (1983)

"Books of the Times" http://query.nytimes.com/gst/fullpage.html?res=9506EFD61038F930A1575AC0A964948260&scp=62&sq=&st=nyt, The New York Times (23 September 1982)

"On Three Ways of Writing for Children" (1952) — in Of Other Worlds: Essays and Stories (1967), p. 25
Context: Critics who treat adult as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow. But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.

Often misquoted as: "I have found that most people are about as happy as they make up their minds to be." or "People are just as happy as they make up their minds to be."
This quote is not found in the various Lincoln sources which can be searched online (e.g. Gutenberg). Niether does Lincoln appear more generally to use the phrase "making up {one's} mind". The saying was first quoted, ascribed to Lincoln but with no source given, in 1914 by Frank Crane and several times subsequently by him in altered versions. It was later quoted in How to Get What You Want (1917) by Orison Swett Marden (Thomas Y. Crowell Company, 1917), 74, again without source. Alternative versions quoted are: "I have found that most people are about as happy as they make up their minds to be" and "People are just as happy as they make up their minds to be."


Source: https://quoteinvestigator.com/2012/10/20/happy-minds/#:~:text=%E2%80%9CPeople%20are%20about%20as%20happy,up%20their%20minds%20to%20be.%E2%80%9D&text=Remember%20Lincoln's%20saying%20that%20%E2%80%9Cfolks,up%20their%20minds%20to%20be.%E2%80%9D

Curiously in later books Crane, e.g. Four Minute Essays, 1919, Adventures in Common Sense, 1920, "21", 1930, Crane mentions other routes to happiness and does not again use this quote.

Marden used a great many quotes in his writings, without giving sources. Whilst sources for many of the quotes can be found, this is not true for all. For instance he mentions another story in which Lincoln says "Madam, you have not a peg to hang your case on"; this also does not seem to found in Lincoln sources.