Two lines, and one of Stevenson's best-known: the world is so full of things that we might as well all be happy about it. Short enough for a child to repeat back after a single hearing.
From A Child's Garden of Verses. See the whole collection.
The world is so full of a number of things,
I'm sure we should all be as happy as kings.
Public domain. Text from A Child's Garden of Verses (Robert Louis Stevenson, 1885), via Project Gutenberg. View the source edition