custom menu for responsive mode

Hello,
I’m using a child theme of Grow and would like to use a different menu in the mobile view. I am almost there but unable to override the function thinkup_input_responsivehtml2() in the child functions.php without getting a fatal error. I tried a few methods that i know of.
Now resorted to editing the parent theme file: admin/main/options/01.general-settings.php in the meantime and it works well.

In the child theme i have:

add_action( 'after_setup_theme', 'register_mobile_menu' );
function register_mobile_menu() {
  register_nav_menu( 'mobile_menu', __( 'Mobile Menu', 'grow-child' ) );
}

The modified function in general-settings line 414:

function thinkup_input_responsivehtml2() {
global $thinkup_general_fixedlayoutswitch;

	if ( $thinkup_general_fixedlayoutswitch !== '1' ) {
// check for mobile
		if ( wp_is_mobile() ) {
			$menu_loc = 'mobile_menu';
				} else {
			$menu_loc = 'header_menu';
				}
//
		$args =  array(
			'container_class' => 'responsive-links nav-collapse collapse', 
			'container_id'    => 'header-responsive-inner', 
			'menu_class'      => '', 
			'theme_location'  => $menu_loc, // <- variable from wp_is_mobile
			'walker'          => new thinkup_nav_menu_responsive(), 
			'fallback_cb'     => 'thinkup_input_responsivefall',
		);

		echo '<div id="header-responsive">',
		     wp_nav_menu( $args ),
		     '</div>';
	}
}