In terms, child (TL:4) refers to a human between the toddler stage (ages 12 months-24 months) and the adolescence period (ages 13–18); in can also refer to an adult, who continues to look at the world through child-like eyes.
The following are quotes respect to childishness:
- “To all rational readers, the use of the chemical theory [in Elective Affinities] is nonsense and childish fooling around.”
- — Christoph Wieland (1810), “Letter to Karl August Bottiger” (note: burn after reading), Jul 16 
The following are related quotes related to understanding or curiosity:
- “There are many questions in philosophy to which no satisfactory answer has yet been given. But the question of the ‘nature of the gods’ is the darkest and most difficult of all. So various and so contradictory are the opinions of the most learned men on this matter as to persuade one of the truth of the saying that philosophy is the child of ignorance.”
- — Cicero (45BC), On the Nature of the Gods (Ѻ)
- “I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”
- “When we begin the study of any science, we are in a situation, respecting that science, similar to that of children; and the course by which we have to advance is precisely the same which nature follows in the formation of their ideas. In a child, the idea is merely an effect produced by a sensation; and, in the same manner, in commencing the study of a physical science, we ought to form no idea but what is a necessary consequence, and immediate effect, of an experiment or observation.”
- — Antoine Lavoisier (1789), Elements of Chemistry (pg. xvi)
- “All physical theories, their mathematical expressions apart, ought to lend themselves to so simple a description that even a child could understand them.”
- Child – Wikipedia.