A well-informed mind is the best security against the contagion of folly and of vice. The vacant mind is ever on the watch for relief, and ready to plunge into error, to escape from the languor of idleness.
Ann Radcliffe, The Mysteries of Udolpho, 1764
What a great way to say that the idle mind, like idle hands, are the "devil's playground". I don't even think that it's necessary that children be involved in highly structured learning so long as they are provided daily opportunities to explore knowledge and the world around them.