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.
About this poem
Confirmed as a 2-line poem, the shortest in the collection, text matches exactly.
Just two lines, and among the most quoted things Stevenson ever wrote for children. It sums up the whole collection in a sentence: that noticing things is enough to be happy about. Good as a closer rather than an opener, said once and left to sit.
Public domain. Text from A Child's Garden of Verses (Robert Louis Stevenson, 1885), via Project Gutenberg. View the source edition