[eluser]WanWizard[/eluser]
If you have this situation, there's a flaw in your class design.
If you want this, define the parent class as abstract, and the method as abstract in the parent class, to force it to be available in the child, and to force a child to be present.
When you do this, you can just use $this->method() in any parent method, which will call the method defined in the child.
In the CI architecture, if you have a common method, you should store this in a library. Also, CI by default does not support controllers calling controllers...