method and it's children organizational confusion

Follow driver_example. Children have access to parent's methods, but not one another's (obviously).

The main driver extends CI_Driver_Library. Children extend CI_Driver. If you want common or semi-common methods and variables among children methods you can use interfaces.

__constructor() should have:

$this->valid_drivers[] = 'Blah_One';
$this->valid_drivers[] = 'Blah_Two';

Don't need to load children. Just load the parent: $this->load->driver('Document');

Structure of files should be:
Base directory: ParentName
File in directory: ParentName.
Directory in base, alongside file above: drivers
Files in drivers:
- ParentName_One - ParentName_Two

Will call children by doing this:
$this->ParentName->One->method();