Categories loop

// WP_Term_Query arguments
$args = array(
	'taxonomy'               => array( 'project_category' ),
	'parent'                 => 11,
);

// The Term Query
$term_query = new WP_Term_Query( $args );

// The Loop
if ( ! empty( $term_query ) && ! is_wp_error( $term_query ) ) {
	// do something
} else {
	// no terms found
}