Accessing properties and methods in parent from child?

How efficient (or inefficient) is it to send the parent class to the child class via argument (ie: child.loadParent(this)) and access the parent that way? I am aware of extending, but whenever i try to extend a parent to a child when the instance of that child is created in the parent i always get an error. what is the best way of manipulating the parent via child?

**I am aware that there are multiple resources for this information online, but the answers are always far to generic.

Thanks.