get_terms of specific parent (including the parent itself)

I don’t think there’s a way you can setup the arguments such that you can get all children of a term and that term in one call, so you’ll probably just have to tack on the parent as a separate call.

Using get_terms for the children, then get_term.

$parent_id = 100; // set as appropriate
$terms = get_terms( array(
    'taxonomy' => 'YOURTAX',
    'hide_empty' => false, // omit if not applicable (used for testing)
    'child_of' => $parent_id,
) );

$terms[] = get_term( $parent_id, 'YOURTAX' );