I think "gingers" are people with red hair, freckles, and pale skin who have some problems with their survival. And for a stepchild, who is a child of an early marriage, is also very difficult to survive having a stepmother or a stepfather. Hence a ginger stepchild might be taken as a symbol of hard surviving. The ginger stepchild might be a concrete red-haired stepchild with a lot of problems.
And please note that, with regard to the contents of post number 2, having ginger hair, freckles and pale skin has absolutely no bearing on your chances of survival. There is also no such thing as a "concrete" child, stepchild or any other type of offspring.