The sandman is a fantasy or folk figure associated with sleep and dreaming. Also, little children sometimes have always a sensation of sand on their eyes when they are sleepy. So the mothers explanation was that Mr. Sandman is coming and then is time to sleep

Wasn't the 1950s version of the song "Mr. Sandman" by the Chordettes featured in the film Back to the Future).

http://heavens-gates.com/50s/mrsandman/
(that link also has their rendition of 'LOLLIPOP' :D Gosh I'm OLD! haha)

The story of the sandman has been borrowed and recast into a number of popular songs, stories, movies, and comic books. The name has been borrowed for wrestling names, song titles, album titles, and several superheroes.