How to Hide the "Child Themes" Admin Menu

function plt_hide_child_theme_configurator_menus() {
	//Hide the "Tools → Child Themes" menu.
	remove_submenu_page('tools.php', 'chld_thm_cfg_menu');
}

add_action('admin_menu', 'plt_hide_child_theme_configurator_menus', 11);

Where do I put this code?