I am wondering how should i insert the category programmatically and it   should insert only 1 time after the theme is activated.

I know up to here.

    function example_insert_category() {
    	wp_insert_term(
    		'Science',
    		'category',
    		array(
    		  'description'	=> 'This is an Science category.',
    		  'slug' 		=> 'science'
    		)
    	);
    }
    add_action( 'after_setup_theme', 'example_insert_category' );

i don't know how to insert children etc. I want to insert category structured like this:

    Science
    --Grade 11
    ----Science
    ----Management
    ----Humanities
    --Grade 12
    ----Science
    ----Management
    ----Humanities

Is it possible to do programmatically?

Thanks