A colloquialism meaning "a really severe beat-down." Apparently it comes from some people's belief that redheads and stepchildren ain't worth much, and if you combine the two, you're getting something that you can safely whallop with maximum force and enthusiasm.

Addendum: smartalix says: "I believe the term indicates more that the child is more visible as 'not mine' due to the red hair, making it a target as 'not really' part of the family."