This quote actually comes from a short essay that Stevenson wrote called "Child's Play," which is an evocation of the state of childhood and a panegyric about the many advantages of being a child. Stevenson argues that children approch the reality of the world around them through fantasy and make-believe, but that as they grow up they exchange this fantasy for the cold, harsh world of abstract realities that seek to do away with illusions and play. Adults, as a result of this, should not be in such a hurry for their children to grow up, and should let them engage in their idle play a little longer. To support his point, Stevenson uses the following quote:
Let them doze among their playthings yet a while! For who knows what a rough wayfaring existence lies before them in the future?
Note the contrast that is created between the use of the verb "doze" and the "rough wayfaring existence" that is associated with adult life. This quote depicts the state of childhood as being tranquil and restful, but in contrast the life of an adult is one of "rough" action and constant movement, without rest, as suggested through the adjective "wayfaring."
I wonder if Stevenson is familiar with and perhaps referring to Wordsworth's Ode: Intimations of Immortality, where he refers to our earthly existence as "a sleep and a forgetting"; "Earth fills her lap with pleasures of her own" etc.