I saw a movie where a family are camping, parents and a son in a rv. They are on a cliff, the son is outside playing, the father is moving the rv and slowly slips and of the cliff leaving the son to fend for himself.
What is it called?

Tough to find based on that scene. Do you know any of the actors names?

Could be The Earthling.

Plot summary from imdb.com (http://www.imdb.com/title/tt0080671/plotsummary):

Patrick Foley has been on the move all his [life]. Tired of drifting, he wants to spend his last days in an isolated Australian valley where he grew up. On his difficult journey he meets Shawn, a little desperate city-boy whose parents were killed in an accident in this remote inhospitable territory. Being unable to accompany the boy back to the civilized world he reluctantly takes him with him on his trip to that valley and teaches him in a rugged way how to survive...

Does that sound like it, dehcho?