A woman backs into a young boy with her car and takes him home. She finds out that he can wish for anything and he wished his original family away and replaced them with new people. In one scene one of the members of his new family (aunt or cousin) drops a note and he finds it, he sees that the note calls him a monster, so he sends her into a cartoon on the tv and kills her. I think at the end of the movie he wishes for everything to go away until the woman who hit him with her car convinces him to wish for his original family back and everything else to go back to the way it was before.
Sounds like it’s from segement three of Twilight Zone: The Movie (1983), “It’s a Good Life”. The boy’s name is Anthony and the woman is Helen. In the end he releases the family, and Anthony and Helen drive off down the road as a field of flowers springs up alongside the road.
The plot summary sounds close but this movie had color.